Conor Cruise O’Brien, who has died aged 91, was a natural controversialist, probably the most pugnacious Irish intellectual since George Bernard Shaw. He was a man of so many contradictions that to call him a blend of all these seems utterly inappropriate; rather, they appeared to pull him in many contrary directions at once. He seems posthumously fated to give rise to further controversy, since opinions on his career, his writing, his personality and his public stances vary hugely.

He was a historian, an essayist, a journalist-publicist, an academic, a politician, a career diplomat, a cabinet minister (for nearly four years), a man who held many plum jobs, yet was constantly at war with the intellectual and socio-political establishments of his time. At times he seemed consciously to stand above the battle(s), yet his attitude to many of his antagonists, intellectual or political, was often personal and he could be vituperative in his verbal attacks on enemies, real or imagined. His contempt for Charles Haughey, twice taoiseach and long-term leader of the Fianna Fáil party, was notorious, and much of it seems to have been returned by Haughey, who refused to engage in public debate with him.

Not even O’Brien’s denigrators, however, could deny that he was an intellectually formidable figure and a man who commanded attention in many countries. Perhaps his nearest equivalent were French intellectuals such as Jean Paul Sartre and Albert Camus. O’Brien, as with many Irishmen of his generation, was deeply influenced by French culture. His early essays on contemporary French writers, especially the neo-Catholic novelists of the 1940s such as François Mauriac, first brought him into the public eye, though they were written under the pseudonym Donat O’Donnell. Later he used the title of a Mauriac novel for his book Maria Cross, which dealt largely with those writers and the intellectual and moral dilemmas with which they wrestled.

O’Brien was born in 1917 in Rathmines, a Dublin suburb, the only child of Francis Cruise O’Brien, a journalist who worked for the Freeman’s Journal and later the Irish Independent, and Kathleen Sheehy, a teacher, feminist, pacifist and author of a book on Irish grammar. His father died when his son was 10, so the dominant influence on O’Brien was his strong-minded mother.

Although his father was an agnostic, O’Brien’s first school was Muckross college, a Catholic convent school. Later he went to Sandford Park, nominally secular but in effect imbued with the Protestant ethos. His mother’s influence had made him fluent in Gaelic, and he won a sizarship to study Irish and French at Trinity College, Dublin. O’Brien was a brilliant student and won a scholarship at the end of his first year, which brought an allowance of £30 a year and rooms in college - a coveted privilege - at half price. His roommate was Vivian Mercier, later a professor of English literature and author of books about Beckett and the Irish comic tradition.

After his mother’s death in 1938, he supported himself by giving “grinds” and by dabbling in journalism. Meanwhile, he kept winning prizes. He was active in the college debating society, edited the college magazine, and joined the Fabian society and the Labour party. In his final year he married Christine Foster, a schoolmaster’s daughter. He graduated with a first and a year later took another first in history.

Incomprehensibly, he failed the Irish civil service entrance examination, but passed on his second attempt in 1942 and joined the department of finance where he spent two years before moving to the department of external affairs (now foreign affairs). In view of O’Brien’s later stances towards Northern Ireland and Republicanism, it is rather ironic to note that in the late 1940s he worked energetically in the Irish government’s campaign against partition. Meanwhile, he tried his hand at writing poetry, which was tactfully rejected by Sean O’Faolain, editor of Bell magazine, but he soon made his mark as a critic and commentator.

When the Irish government decided to set up its own Irish News Agency, O’Brien became its first managing director, staying for the seven years it existed. Under a new inter-party government that replaced De Valera’s Fianna Fáil, he was sent as a councillor to the Irish embassy in Paris. At this time, Irish governments, following the lead of France, were promoting writers and intellectuals to the diplomatic circuit.

With the return to power of Fianna Fáil, O’Brien for a time had a good working relationship with the veteran minister for external affairs, Frank Aiken. Ireland by now was playing an active and vocal role in the UN assembly, and O’Brien was generally credited with being one of the people who formulated its policies - which included bringing on to the agenda China’s admission to the UN, to the chagrin of American diplomats.

O’Brien was in his 40s when he entered into the most fateful and controversial chapter of his life - his posting to the Congo (now the Democratic Republic of the Congo) by the direct initiative of Dag Hammerskjold, then secretary-general of the UN. He arrived in Elizabethville, the capital of Katanga province, in June 1961 to find himself in the epicentre of an international hotbed. The secession of Katanga, the murder of the first prime minister Patrice Lumumba (often ascribed to European-paid mercenaries), the dubious role of Union Minière, made the front pages of the world press for months on end. In the end O’Brien, apparently acting on what he thought was a UN resolution, ordered the UN peacekeeping force into action against the mercenaries and against Katanga’s secession. The crisis became semi-farcical when the poet Máire MacEntee (daughter of a Fianna Fáil minister) arrived to join him and declare her support.

He had stirred up a hornets’ nest internationally. One of the most vocal critics was Paul-Henri Spaak, then Belgian foreign minister and now remembered as an architect of European unity. “Who is Conor Cruise O’Brien?” asked Harold Macmillan, and answered his own question: “An unimportant, expendable man.” Pressures on him, on the UN and on the Irish government multiplied. Hammarskjold was forced to desert his protege, then died in a plane crash and his successor, U Thant, formally agreed to a request from Aiken that O’Brien be released from further UN duty. Almost immediately, he announced his resignation from Irish government service.

He had become a hot potato, and the controversy continued when he later published his version of events in the book To Katanga and Back. Meanwhile, he divorced his wife and married MacEntee, and they adopted two Congolese children.

However, the Congo chapter had earned him at least one admirer - President Kwame Nkrumah of Ghana, who invited him to become vice-chancellor of the university there. For a time this relationship went well, but the two drifted apart and O’Brien - never a man to stay silent long - was occasionally outspoken about what he felt were local breaches in civil rights and free speech. Nkrumah, becoming somewhat paranoid after an assassination attempt, deported several of O’Brien’s European colleagues at the university. After an interval, O’Brien followed them voluntarily, becoming Albert Schweitzer professor of humanities at New York University.

There he became a vocal critic of the Vietnam war, attending protest marches. In one, he was kicked by a policeman, and was in considerable pain for days afterwards. During this period, his play Murderous Angels was staged in Los Angeles and later had a brief run in New York - allegedly after black militants had brought pressure on some of the actors to pull out.

Eventually he returned to Ireland, where there had been a swing to the left politically, and stood as a Labour candidate in a Dublin constituency. Unexpectedly, he won a big vote and became embroiled with his bete noire, Haughey, for whom he later coined the word Gubu - standing for “grotesque, unusual, bizarre and unprecedented”. O’Brien attacked Haughey over his role in the arms crisis, a cause celébre at the time, in which certain Fianna Fáil politicians were alleged to have tried to smuggle arms shipments to nationalists in Northern Ireland.

O’Brien became minister for posts and telegraphs in a coalition government (Fine Gael-Labour). His anti-republican line resulted in the amendment to section 31 of the Broadcasting Act that virtually denied active or even strongly vocal republicans access to radio or TV channels. Dublin journalists were sharply divided about the measure, and it did seem unbalanced considering the frequent interviews with unionist politicians - including Ian Paisley - that were part of Dublin coverage of events in the north. The section has since been repealed. O’Brien believed that there was too much sympathy for Sinn Féin in Radio-Telefís Eireann in particular - a view which, in retrospect, is hard to substantiate. His stated view was: “If the Provos are successful, there will be civil war into which the south will be drawn.”

O’Brien had little direct experience of politics or even of life in Northern Ireland, yet he was consistently vocal on the subject and became his party’s spokesman on the north. To many, his attitude seemed not only anti-republican - which, considering his essentially home rule background, was to be expected - but anti-nationalist in any sense, and arguably his stance made things harder rather than easier for men such as John Hume, who was striving to maintain a moderately nationalist line, as an alternative to IRA violence. When told that his pronouncements - widely reported at home and abroad - were close to official London policy, O’Brien retorted: “Yes, I am pro-British. I am also pro-French and pro-American. I am even pro-Russian in that I am pro the Russian people. But I am more pro-Irish than I am any of these things. Ireland is my country, and I am just as Irish as any bloody IRA man.”

Time was running out for the coalition, and in the 1977 election Fianna Fáil came back to power and O’Brien lost his seat. He was elected to the Irish senate, but more to his inclinations was the job as editor-in-chief of the Observer in London, to which he was appointed in 1978. The following year he received Granada TV’s columnist of the year award for his journalism and in 1984 he won the Channel 4 What the Papers Say award for his work for The Irish Times and the Observer. Of his career as a politician, he said that his election defeat had liberated him from the necessity of saying things he did not believe: “It sickened me, and I am glad to get out of it.”

In 1987 he was embroiled in controversy again, this time as a result of a visit to South Africa, where his lectures at the University of Cape Town angered black students. However, as his writings show, O’Brien was no friend to racial (or racist) policies, and his 1986 book on Israel, The Siege, was characteristically independent in its viewpoint.

Of his later writings, undoubtedly his most important book was the biography of Edmund Burke, The Great Melody - a phrase borrowed from Yeats. A degree of autobiographical self-identification with Burke was noticed by several commentators.

O’Brien continued to denounce nationalism vehemently: “Nationalism everywhere tends to be xenophobic.” He also attacked the “insidious and demoralising peace process” in the north, and he shocked some of his liberal followers by canvassing for a Unionist candidate in North Down. Through all this, however, he continued to live peaceably with his second wife in their home in Howth, north of Dublin. In 1992 he told Fergus Pyle of The Irish Times: “I have never spent an entire year away from Ireland. In every year of my life I have been able to come back here for some part of the summer - what the lawyers call the animus revertandi. It is quite strong.”

Many still believe that O’Brien was at his best as an academic historian, and that the book Parnell and his Party, which grew out of a student thesis, is his most valuable work. A Concise History of Ireland, published in 1972 under the joint names of himself and his second wife, is a useful “potted” survey and has gone through several editions. His essays and occasional pieces also contain some excellent literary criticism, without the contemporary polemics that intrude into so much of what he wrote and said. But then, polemics and O’Brien could seldom be kept apart for long, and public controversy and debate seemed to answer a fundamental demand of his nature.

O’Brien was a maverick, both as a writer and politician, and to accuse him of inconsistency is beside the point. Some shrewd analysts viewed him as a fine intellectual led astray into public life by ambition and the desire to prove himself a man of action. Others saw him as a courageous radical nonconformist who challenged the forces of obscurantism. It is certainly arguable that, like Burke, he began as a Whig radical and ended up as almost a reactionary. The warring aspects of his personality partly had their roots in old Parnellite and home rule politics, which personalised and sometimes embittered public debate, partly in the example of the generation of French intellectuals that emerged after the second world war, partly in the liberal leftism of the 1950s, and partly in 1960s protest politics. But the crucible of all these was his own mercurial, restless personality and intellectual brilliance, backed by a historian’s sense of history and the born journalist’s flair for being in the wrong place at the right time.