Words became important to writer Jennifer Johnston at an early age; words and the stories those words could shape, would shape. As a child she was aware of her mother, an actor, learning lines. She also grasped the edifying, if at times unsavoury, allure of secrets. Most of all, she discovered the power of truth. Her intelligent, ironic, almost neutral fiction of voices deals in truths, rarely pleasant - but as she has often said herself, ‘living isn’t easy’. The daughter of playwright and broadcaster Denis Johnston and actor and director Shelah Richards, Johnston began her lifelong study of human behaviour by watching her parents end their marriage when she was eight years old and her brother, Michael, was three. Her father went away to war and, for her, never really came back.

No, living is not easy. In 2000, on the publication of her 12th novel, The Gingerbread Woman, Johnston calmly told me she was a realist: ‘I see life as a very tough business.’ She will celebrate her 80th birthday on Tuesday, and for almost half of that time she has been a novelist, publishing her first novel The Captains and the Kings in 1972 when she was 42.

It was quickly followed by The Gates (1973), a novel of which she has been critical, and then came How Many Miles to Babylon? (1974): it is a fine work and the culmination of her interest in the Great War as an enduring theme for her. The young Johnston had known men who had served in it and others who had lost fathers and brothers. Those first three books, begun in her head during her late 30s and written in quick succession as if making a concerted burst through a door that had been locked for too long, taught her how to write while, on a public level, establishing her as a writer.

Unlike most writers there was nothing autobiographical about them. She has never tended to write about herself. Instead, she watches and listens, taking up bits and pieces about people she has known acting out their lives in the middle-class world she knows. During her time at Trinity College Dublin studying English and French, she often felt she made her fellow students uneasy through this habit of watching which was no doubt intensified by Johnston’s tendency, the legacy of poor eyesight, to peer.

Parties relaxed, she felt, once she and her gaze left. Her narrators are also watchers, dominated by their inner worlds. She did not complete her degree. She didn’t enjoy university, yet could see that the Trinity of those years attracted an interesting cross-section of students such as ex-servicemen and wealthy English youths who had failed to enter Oxford and Cambridge. All the while, Johnston was storing up the gestures, the nuances that would feed her interest in details and the voices in her head that would eventually become public through her books.

Her fiction has explored our ability to make mistakes. Above all, she looks at the small hurts that render us emotionally remote. Her novels are also suspended between the several Irelands defined by social, political and cultural differences.

Johnston’s sophisticated, at times deceptively conversational, narratives have drawn on social class as it exists in a country caught between the contrasting Catholic and Protestant cultures. As a writer she is more effective through nuance than open comment, as in Shadows on our Skins, which openly confronts the Northern conflict - yet this was Booker shortlisted in 1977, while some of her more deserving novels were overlooked. It is also ironic that her work is being taught at US universities - although she does not currently have a US publisher. If Elizabeth Bowen has a literary heir, it is Johnston who, despite superb novels such as the early war novels and The Christmas Tree (1981) and The Illusionist (1995), remains consistently underrated, suffering to some extent because of the immense achievement of William Trevor. Yet more than any other Irish writer, it was Johnston who took the Big House novel, with its final vestiges of fading privilege, out of the countryside and towards its inevitable, and logical, resting place - the more narrow, less romantic, and ultimately realist suburban comforts of Dalkey and Killiney.

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Although not born into Big House stock, her second marriage would introduce her to this; Johnston was an upper middle class child who grew up in Donnybrook. After her father left - and, for a while, Johnston did think he would return - she lived with her mother and brother in a top-floor flat in a rambling house, Greenfield Manor, set in a large garden off Nutley Lane in Dublin 4. She attended Park House, a Protestant girls’ school, which once stood at the top of Morehampton Road. As this period coincided with the war years, many of the girls were English, sent over to a neutral country. It was easy to feel privileged because, such were the inevitable shortages, no one had anything.

Johnston, it seemed, was destined to move between the Irish and the English, without taking sides. Yet she has no doubt that she is an Irish writer and very much a Dubliner. In 1951, at 21, she married Ian Smyth, a fellow student at Trinity. They lived in Paris for a year before moving to London. It was an exciting time: post-war London was recovering, rebuilding itself. It was an interesting first act - marriage and four children and living in London. Johnston is well known for her candour; she is blunt, practical, opinionated and slightly theatrical with her sweeping hand movements and vividly Anglo-Irish turns of phrase.

In an interview situation she makes sure the interviewer is aware, that she, the interviewee, is conscious that an interview is exactly that, not a conversation. Yet once that formal distance is acknowledged, she doesn’t evade questions, she is too honest and makes no secret of the fact she feels her writing for the theatre has been ignored. Johnston freely admits she is self-obsessed. Most people are - we just don’t say it. She is a good talker, brisk, emphatic, graced with a distinctive speaking voice with its mixture of Dublin, London, Derry and the broad vowels of the English west country - an accent her mother accused her of inventing. As part of an RTÉ radio series, A Giant at My Shoulder, she gave a personalised, non-academic lecture on Shakespeare and spoke with such passion it was as if she were sitting beside the listener.

The impulse to write was there long before she actually began. The girl and young woman who lived largely in her head was all too conscious of her imposed role in life as her father’s daughter, her mother’s daughter, her brother’s sister, her husband’s wife, her children’s mother. Her need to establish herself had some part in the ending of her marriage to Smyth, a lawyer, who later remarried and lives in Bath. When Johnston married David Gilliland she went to live with him at his family home, Brook Hall, a curious Georgian structure - part lighthouse, part squashed birthday cake - just outside Derry. It is famous for its arboretum, which was planted in 1920 and is cultivated by Gilliland, a solicitor and dendrologist, whose family has lived at Brook Hall since 1852.

Johnston makes no claims to being an expert on trees. No, she lives in her head, and is a shrewd, careful and self-confessedly, emotional reader. If she likes a book, she likes it. Aside from her love of Shakespeare, she will, on reading a book she likes, immediately tell everyone about it. I remember receiving a note from her, applauding an article about the Scots-Canadian Alistair MacLeod. Johnston is generous to, and protective of, other writers and was an early mentor to Sebastian Barry. Deirdre Madden’s Orange Prize contender, Molly Fox’s Birthday (2008), bears obvious echoes of Johnston’s stylistic influence.

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Acting as a career, and also as a motif, is central to her fiction; the child who watched her mother battling with scripts has never lost her fascination with performance. Richards had difficulties learning lines. Johnston’s new novel - her 16th - Truth or Fiction (2009), partly narrated by a journalist with problems of her own, looks closely at the antics of a long-divorced couple who sustain a friendship in spite of the old playwright Desmond’s jealous second wife. The first wife, flighty, charming and utterly selfish, was a famous actor. The characters are obviously based on Johnston’s parents, a deduction she enjoys denying.

Far more important is the novel’s variation on her familiar themes of hatred and betrayal. This time, regret is the prevailing emotion. Behind the apparent Peter Pan charm of the playwright’s first wife, Pamela, lies a cold detachment. Mothers and daughters feature throughout Johnston’s work. Her relationship with Shelah Richards was affectionate, although in an interview with me in 1995 she, with characteristic bluntness, described her mother as ‘a spoilt, younger child with a very stormy personality. She expected everything to revolve around her.’ Her mother had been generous with her time to her friends, but less so to her children.

Awkward mother/daughter relationships prevail. In Two Moons (1998), three generations of women - a daughter, a mother and a grandmother - are juxtaposed. Two of the women are still daughters; two of them are still mothers. In The Illusionist, Johnston, a writer ever in control of emotion, writes one of her most heartbreaking scenes: Stella, the sympathetic narrator and a writer, waits for the arrival of her daughter and thinks ‘I haven’t known her to cry since she was a child. To be quite honest, I have hardly known her since she was a child.’

There is wit; Johnston is wry, rather droll, but she never offers the easy response. It is unnerving when she disputes the possibility of happiness. None of her characters is happy. Equally, she will admit to being lonely. It is the fate of those who live in their heads. In The Christmas Tree, Constance Keating, cool, intelligent, likeable, calmly revisits her life as she waits for death. It is a great novel because in its matter-of-fact way it generates intense emotion. This is what Johnston can do - take the reader by surprise. She does it in person as well.

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Perhaps no matter how fast we run, we never run away from our parents. Jennifer Johnston does not seem to have run from them, they ran themselves. Yet in their old age she finally came to know them. When Johnston says of her father, ‘I was his only daughter’, it is charged with unexpected emotion, as is her memory of her mother, who died only six months after Denis Johnston, and asked for forgiveness shortly before her own death. Johnston’s world has been one of restrained emotion; there was a great deal of talking but little outward emotion. All the feeling, however, the anger, the tears, the rejection and the resentment, is in the fiction. Her novels are short, concise, tend to say a great deal behind the banter, make many surgical incisions and niggle away at the memory. Of the multiple ironies about Jennifer Johnston is her shortsightedness: this is a writer who has observed intently and has missed very little.