Only live to be 80, Evelyn Waugh once said, and you may be assumed ‘into that odd circle of ancient savants and charlatans whom the Sovereign delights to honour and the popular press treats with some semblance of reverence’. It’s a good description, of the way that longevity forgives all; ogres mellow into icons, and the fiery rebel ends his days as a national treasure.

But Conor Cruise O’Brien is an exception to many rules. At 85, he is certainly not enjoying a tranquil old age amid universal reverence and honour. A man who once plunged into a succession of snakepits - African civil war in his 40s Dublin politics in his 50s, Fleet Street in his 60s - might have chosen comfortable retirement. Instead, in his 80s, he plunged into one more, in Northern Ireland, and in a way - standing as a candidate for Robert McCartney’s “anti-agreement” UK Unionist party - that astonished even those who knew his implacable, decades-long hostility to Irish republicanism, and dismayed some for whom ‘his influence had once had enormous value’, as the Dublin writer Fintan O’Toole puts it.

He is quite impenitent. ‘I have no regrets,’ O’Brien says - not about any of his bewilderingly various careers, and certainly not about his final career in Northern Ireland. Today, the anniversary of King Billy’s victory at the Boyne in 1690 - a key date in the Orange marching season - finds David Trimble in a more ragile position than ever, O’Brien views the possible disintegration of the Belfast agreement and ‘peace process’ with undisguised pleasure: ‘I’m glad to see this bloody thing crash. It’s been a horrible fraud.’ No less gratifying to him are other dégringolades : the scandals that have beset the Catholic church in Ireland, and the humiliation, amid a different kind of scandal, that engulfed his most bitter enemy, Charles Haughey.

All of that is to think of O’Brien merely in Irish terms, and he is indeed very Irish: in upbringing, in appearance, in humour, even in literary manner. But he’s also a true cosmopolitan, a ‘public intellectual’ of international stature and renown, whose first great fame came in 1961, when the Katanga imbroglio prompted Harold Macmillan to ask irritably: ‘Who is Conor O’Brien?’ Plenty of answers could be given to the prime minister’s question: diplomat, proconsul, American professor, Irish cabinet minister, London editor, agitator; and along with all of those, a writer with a large and truly distinguished body of work to his name. So who is O’Brien?

To begin with, he is a Dubliner, born 18 months after the 1916 Easter Rising and the day after the Balfour Declaration on November 2, 1917: both dates deeply significant in his own story. His family came, he says, from ‘the lower end of the educated middle class’, with the O’Briens, Skeffingtons and Sheehys forming a notable clan within that emergent Catholic bourgeoisie that had moved rapidly upwards well before the free state was established. His father, the gifted but not very prosperous journalist Francis Cruise O’Brien, who died when Conor was young, was a friend of Yeats’s; and to belong to that circle at that time was to stand a good chance of literary immortality. Conor’s maternal grandfather was the Home Ruler ‘Mr David Sheehy MP’, whose wife converses with Father Conmee in James Joyce’s Ulysses ; his mother Katherine Sheehy, a teacher, was the model for ‘Miss Ivors’ in Joyce’s great story ‘The Dead’.

Even though a great-uncle was a prominent nationalist priest, the family sent Conor to a Protestant school, and then (in defiance of an episcopal ban) to Trinity College, Dublin, where he was both brilliant and convivial. But when he graduated ‘there was no welcome for me in academia’. Well after independence, “TCD” still remained a bastion of the Protestant Unionism with which the young O’Brien was then entirely at odds, and he was taught by people who barely concealed their fastidious distaste for Catholics, ‘even an ex-Catholic like myself’.

He had ceased to be a believing or practising Catholic as soon as he grew up, and he regards it as a matter of some pride that he made a career in public service without concealing that fact, in an Ireland that until quite recently was deeply deferential to the Church. Thirty years ago, he was the only declared agnostic ever elected to the Dail, and whatever his other evolutions, his anti-clericism has remained consistent.

He became a civil servant, and spent two decades in the small Irish foreign ministry. For a while he worked under the fiercely republican Sean MacBride in the postwar Dublin ‘anti-partition’ campaign. He now looks back on that time with some shame, not so much at the campaign itself - though he regards it in hindsight as ludicrous, addressed to southern Irish and Irish-American opinion rather than to Northern Ireland or England - as the way that he was for a time, in his words, ‘a successful sycophant’, an accusation rarely levelled at him even by his many enemies.

He was also making a name as a writer and historian, with his book Parnell and His Party (based on his PhD dissertation), and as a critic, with Maria Cross, a collection of essays on Catholic writers, including Charles Peguy and Graham Greene, originally published under the pen-name “Donat O’Donnell”, which he used while in public service. These books led almost comically to further promotions. When Frank Aiken, another tough nationalist, became Irish foreign minister, he took up O’Brien because ‘he’d read my book on Parnell and said he liked it very much’. Aiken was not its only admirer. The book was welcomed as a distinguished work that cast a new light on that crucial passage in Irish history. It also had witty footnotes. (When Parnell was cited in the fatal divorce case, the “addiction to the tu quoque” among his supporters ‘would not have helped the cause much either. In a London church when a clergyman at this time condemned Parnell’s moral lapse, an interrupter loudly asked, “What about the Prince of Wales?”’)

And so O’Brien went to Paris, and then to the United Nations in New York. In 1960, the Belgian Congo became independent under Patrice Lumumba, and was rapidly engulfed by violence. The southernmost - and much the richest - province of Katanga seceded under Moise Tshombe, none too indirectly backed by Belgian mineral interests. After Lumumba was overthrown and then murdered, O’Brien was sent to head a UN peace-keeping mission to Katanga. He was chosen not so much for his diplomatic skills as because, in another wry twist, Dag Hammarskjöld, the intellectual and introspective Swedish secretary-general of the UN, had read and admired Maria Cross.

Quite how far O’Brien’s close reading of Catholic writers was a qualification for dealing with strife in central Africa was an interesting question, and in any case the unpropitious adventure soon ended grimly. Hammarskjöld was killed in a mysterious air crash, while O’Brien was vilified in the London Tory papers - for whom the UN was ‘the Red Army in blue berets’ - and forced to resign. His revenge on a British government ‘which had become very sensitive about me’ and, according to O’Brien, done what it could to get rid of him, was to write a book, To Katanga and Back, telling as much of the truth as he could about the affair. He later expressed some disillusionment with African nationalism, but that book (as well as a subsequent polemical study of Camus) firmly established his anti-colonial credentials. The truth had not been told before about how far western governments had striven to destabilise Lumumba’s new-born state and O’Brien was one of the first to illuminate this murky episode, writing with an acerbic tone that was no doubt strengthened by his own sense of injustice at the way his career had been ended.

At least Katanga had made him a hero in Africa, and by way of consolation prize he became vice-chancellor of the University of Ghana. It had also brought a climacteric in his personal life. O’Brien had been married young, to Christine Foster, from an educated Ulster Presbyterian family, which gave him first-hand experience of another Ireland unknown to most Dubliners. They had three children, Donal, Fidelma and Kate, but the marriage petered out, and he then met Maire Mac Entee, also an Irish diplomat, daughter of one of President Eamon de Valera’s closest political allies, and a distinguished Gaelic poet. To the astonishment of the press corps covering the story, she joined him in Katanga. Caught up in the violence that engulfed the province, with hundreds killed by mercenaries and lawless gangs and the UN a particular target, they were lucky to escape unharmed. They married when he was free (after a Mexican divorce - there was none available in Ireland, then or for decades after).

They adopted two children in Ghana, Margaret and Patrick, but despite his affection for the place, O’Brien soon became disenchanted by the corruption of Kwame Nkrumah’s regime, and left with quiet relief for several years in a New York University chair, ‘where I was very happy’. He was also still very radical, maybe even radical chic, a self-proclaimed socialist, anti-colonialist and ‘anti-anti-communist’, demonstrating against the Vietnam war and enjoying an exalted reputation on the left, which seemed to be his natural home.

And yet, as well as a restless streak, O’Brien has always had a pronounced esprit de contradiction. Having called the missionary Albert Schweitzer ‘a tragic anachronism’ who embodied ‘the most noxious aspects of the white man in Africa’, he shortly afterwards became Albert Schweitzer Professor of Humanities at NYU; having said the Irish Labour party had been dominated for years ‘by dismal poltroons, on the lines of O’Casey’s Uncle Payther’, he returned to Ireland to be elected to the Dail as a Labour member. To make his political debut more piquant still, under the Irish system of multi-member constituencies, he shared the Dublin North-East seat with one Charles Haughey of Fianna Fail.

His homecoming coincided with the explosion of violence in Northern Ireland, the issue that was to determine much of his life thereafter. In 1972, he published States of Ireland, which set out to demolish patriotic mythology. As O’Toole says, he now seemed more formidable than ever, eloquently denouncing the republicans and all who supported them. ‘And he was also much clearer than anyone else,’ O’Toole remembers, in asking pointed questions about Haughey’s murky finances.

By the time Labour joined a coalition government in 1973, O’Brien had made one more enemy. John Hume, a founder of the Social Democratic and Labour party and founding member of the civil rights movement, was regarded as the oracle of ‘the North’ in Dublin, though not by O’Brien. ‘I was originally intended to be foreign minister but,’ he believes largely because of his antipathy for Hume, ‘I didn’t get it,’ to his considerable resentment. Instead he was made minister of posts and telegraphs, which gave him the opportunity to ban Sinn Fein from Irish airwaves, thus adding further to his growing unpopularity in Dublin.

What also vexed his enemies was his literary eminence, which put him in a quite different class to other politicians. How many members of the present Irish cabinet, or indeed the British or American cabinets, could be imagined writing brilliantly on Michelet, Mauriac and Camus? He is a scholarly rather than an academic critic, who was never likely to have much sympathy for what he once called ‘deconstructionism, post-structuralism and what-not’.

One of his collections of essays was published as Writers and Politics, and this is really his forte, situating and examining writers in their social and political context. He has occasionally done this in a partisan way - he is one of those whom Christopher Hitchens scolds for obtuse unfairness towards Orwell - but is at his most fair-minded and penetrating with his fellow-Irish, from Somerville and Ross to James Joyce. Not that he has ever much cared for Joyce despite the family connection. He recognises Joyce’s genius, of course, but says that he has always found the man who depicted himself in A Portrait of the Artist as a Young Man a tiresome prig. O’Brien’s real heroes are Edmund Burke, as both writer and thinker, and WB Yeats. O’Brien’s long essay ‘Passion and Cunning’, written to mark Yeats’s centenary in 1965, is still widely regarded as one of the best things ever written about the poet and one of the best introductions for anyone who wants to understand 20th-century Ireland.

When the coalition collapsed with the 1977 election, O’Brien was chagrined to lose his own seat. Musing about this some months later at a Spectator lunch in London, he said that ‘if the voters were against me personally I don’t think it was because of my stand on the IRA but more likely the telephones’, to which the journalist Mary Kenny added, ‘and God forgive me for saying so, Conor, but you can’t blame them’ (anyone who tried using the Irish telephones in the 1970s will understand this exchange). Now yet another career beckoned, when to everyone’s surprise he was made editor-in-chief of the Observer in 1979, and spent several exhilarating if exhausting years in London.

Colleagues like Simon Hoggart remember him with much affection, both for tremendous columns and tremendous sessions in the pub. He was very good and very droll company, most of the time. ‘Conor would reach a certain point in the evening, when his voice became much higher-pitched, and then another, when he’d start addressing you as ’Party Comrade’.’ On the pavement outside El Vino’s after one such long evening, O’Brien mentioned his son Donal. Hoggart said he knew him: ‘He’s a very nice bloke.’ At which ‘Conor almost picked me up my my lapels and said very deliberately ’ I... know... that’.’ He has also been known to say in the same voice, ‘Three doctors have told me that if I did not stop drinking I would die. All three are... themselves... now... dead.’

His reign as editor-in-chief was not without problems. There was a sharp row with the National Union of Journalists when he determined to get rid of Mary Holland for her republican sympathies. The paper’s political columnist was Alan Watkins. Remembering how O’Brien had made his name in bien-pensant circles 20 years earlier, Watkins was surprised by his new incarnation, and now led the prosecution: ‘My Vishinsky,’ O’Brien sardonically calls him (they remain friends). As a columnist, O’Brien was generally regarded as a great success, witty, erudite and almost at his best when he strayed from the politics of the week. Anthony Howard, deputy editor at the time, recalls him as a trouper: ‘He covered the 1983 election brilliantly.’

But it was not a happy time in the Observer’s history. First it was bought by an American oil company, Atlantic Richfield, which imported O’Brien for his intellectual cachet, and then it was sold to Lonrho and the notorious Roland ‘Tiny’ Rowland. O’Brien thought Rowland totally unsuited as an owner of the paper and he said so. After that his departure was a matter of time. Donald Trelford, then editor, says: ‘Conor was his own worst enemy’ in the way he handled office politics, although, ‘he’d done enormous good for the paper and was a great loss’ (they too remain friends).

O’Brien returned to Ireland, and to books, as well as journalism, and regular forays to American campuses. He claims not to feel as isolated as is widely supposed; he moved ‘in the very best of company’, sustained by his ‘little platoon’. That phrase is from Burke, the subject of The Great Melody, one of O’Brien’s best books, and it’s the caption to a wall of family photographs in his house.

His life has known one deep sorrow with the sudden death from a brain haemorrhage of his daughter Kate, a writer and publisher, six years ago at the age of 49, but the rest of the family flourish. Donal is a professor at the School of Oriental and African Studies in London, Fidelma is married to Nicholas Sims, a businessman (and son of a Church of Ireland Archbishop of Dublin), Margaret works for a computer company in Navan, and Patrick, having cut a dash in high-tech communications in Dublin, is now in London hoping to do the same. At O’Brien’s 85th birthday lunch last year, the little platoon gathered; Maire, Donal, Fidelma, Margaret and Patrick, along with assorted consorts and most of O’Brien’s five grandchildren.

After savaging Thomas Jefferson in The Long Affair, O’Brien is working on a book about the presidency of George Washington, his American hero. He is more subdued now, after suffering a mild stroke in 1998 while engaged in Northern Irish politicking. But it has not softened his stance, which continued to be ‘particularly distressing for people like me’, says O’Toole. ‘He’d taught my generation not to accept the self-serving myths of either tribe in Ireland. And now there he was in Belfast helping to promote one of those myths. Instead of the duty of a public intellectual to be equally critical of both sides, he was explaining himself away with the kind of argument he’d have torn to shreds if it’d come from some Provo fellow-traveller.’

In O’Brien’s own view his new role was a logical development from earlier positions. He had always abhorred republican violence and came to the conclusion that democratic self-determination applied as much to the Protestant Unionists who are the majority in Northern Ireland as to the Catholic nationalists who are the majority in the rest of Ireland. And he had had one particular moment of revelation. During the 1969 violence that threatened Belfast Catholics, one Dublin politician after another said with almost audible italics that ‘we must not stand idly by while our people are in danger’. But, he says, according to the official rhetoric of republicanism, all those who live in ‘the island of Ireland’ are equally ‘our people’, Protestant and Catholic alike.

All the same, his logic led him into strange company. ‘I like and admire Bob McCartney,’ he says of the Belfast QC leader of the UK Unionist party and an MP in the last parliament. He has also shared a platform with Ian Paisley. O’Brien continues to advocate internment ‘for both sets of murderers’, regarding the ‘loyalists’ with as much hatred as the IRA; calls talk of republican splits largely a ruse; and thinks Trimble has been ‘very incompetent’. While enraging his supporters by sitting down with Sinn Fein, Trimble has, says O’Brien, annoyed the southern Irish by his ‘silly attacks’ on the republic.

O’Brien believes the British government is scared of the IRA and is bending over backwards trying to propitiate Sinn Fein. He argues that the circle cannot be squared for good, in that there is no ‘political solution’ available that is permanently acceptable both to the Unionist majority in Northern Ireland and to militant republicanism. He also believes the agreement and executive can’t be resuscitated. But O’Brien has already made precise predictions before now, about political collapse and returning civil war, that have not come to pass.

His knack for taking reasonable premises to extreme conclusions also led him from attractive philosemitism to fervent Zionism and intransigent support for Israel, horrifying an erstwhile admirer, Edward Said. This had painful consequences 20 years ago when O’Brien wrote an Observer column stoutly defending the Israeli incursion into Lebanon. It appeared just as news had broken overnight of the massacres of Palestinians in the Sabra and Shatila camps in September 1982. To say, as Howard does, that ‘we had egg all over our faces’ seems an understatement, but O’Brien was unabashed and persevered to write The Siege, a learned but partisan history of Zionism and Israel, making little attempt to see the Palestinian side or even to consult Arab sources. Maybe this is what Ferdinand Mount means when he calls O’Brien a man ‘whose function it is to be gloriously wrong’.

Still, even those who think him wrong, or who, like O’Toole, are perturbed by the way this one-time idol of the left is now a darling of American neo-conservatives, can never accuse their ‘lost leader’ of having left for a handful of silver or a riband to stick in his coat. In Northern Ireland, he might be mistaken, but there’s nothing in it for him. If he is sometimes self-destructive, he has never been self-seeking.

And whatever the outcome in Ireland, he is surely entitled to be gleeful at the fate of one old enemy. O’Brien hated Haughey partly for his ambivalent attitude to armed republicanism, and partly for the fact that - as many in Ireland appeared not to notice - with every year Haughey spent in politics, he grew flagrantly richer, until the exposure of his financial dealings and his downfall in 1992 forced the Irish ruefully to acknowledge the part played by corruption in Irish public life. This helps explain why O’Brien, who at the time of the 50th anniversary of the Easter Rising in 1966, still wrote about it with admiration, now says: ‘I have ceased to be an Irish nationalist.’ And the Haughey story certainly explains why he told a friend some years ago, ‘It is not pleasant living in a country whose political culture you despise.’ His house bears physical witness to his own honesty. Although crammed with books, and with a breathtaking view across Dublin Bay, it’s a modest enough abode: none of Charlie’s grand old Georgian mansions for ‘the Cruiser’.

Whatever happens in Northern Ireland, O’Brien won’t be judged merely by his political life. His friend and colleague McCartney paid a birthday tribute last autumn, praising O’Brien’s intellect and erudition, but also his moral courage for sticking to his beliefs ‘against the current of popular sentiment and prejudice enjoyed by lesser men’. And he said ‘he is not just a great Irishman, but a great citizen of the world’.

An old Irish saying, roughly translated, holds that ‘Talk doesn’t last - ink does’. Conor Cruise O’Brien has written a dozen books to illustrate that truth and to ensure that, when the bitter controversies that have attended him right until his defiant old age have faded from memory, he will still be read.