For many years she has strenuously resisted the notion of a memoir. She told David Norris in an interview recently that she couldn’t possibly write a memoir - “I’d have to tell too many lies”. She has also said as much to us, her family. But she, the writer Jennifer Johnston, or Ma - or “J” for the purposes of this article, as I can’t bring myself to call her either of the alternatives - has occasionally found other ways to write about her life and family.

Back in 1980, the clan had gathered loyally and in large numbers at the Peacock Theatre in Dublin for the opening of her one-act play The Nightingale and Not the Lark. To our horror, on the stage May Cluskey conjured up a vision of an aged, faded actress who bore an uncanny, though somewhat unkind, resemblance to J’s mother, Shelah Richards, a distinguished Abbey veteran and RTÉ drama director. She was also in the audience with steam coming out of her ears. “Pure coincidence. Nothing to do with Shelah,” J told all who would listen amid the unfolding off-stage drama, also unfairly blaming Cluskey for an unfortunate interpretation of the text.

Now J has done it again, though most of the dramatis personae are no longer in a position to complain. “This is not about my father,” she told her elder daughter, Sarah Smyth, as she presented the manuscript of the new novel, Truth or Fiction, to her.

A couple of years ago she teased readers with the Magritte-inspired title This is Not a Novel and, later, Grace and Truth, both exercises in exploring family secrets and lies and the way they play out over time in the dynamics of family life. Now, with Truth or Fiction, we have, protestations to the contrary notwithstanding, the non-memoir - a picture at one moment in time of her father, the playwright, broadcaster and academic Denis Johnston, and the women in his life: two wives, a mistress, a mother and a daughter.

Like Denis Johnston in the late 1970s, Desmond Fitzmaurice, in Truth or Fiction, is an ageing “writer of plays, war correspondent, literary giant of the thirties” and now “no-one reads his books any more, no one puts on his plays”. Both lived for a time in a splendid house on the end of Sorrento Terrace in Dalkey, in south Co Dublin, looking out over Killiney to Bray Head. I can still see in my mind the distinguished old man, stooped but still agile, leading me through the wide hall into “a high room with two long windows giving out over the bay. The walls were covered with books, floor to ceiling, and the mantelpiece was weighed down with photographs”.

Denis, like Desmond, led a complicated personal life that he obsessively recorded at length, and with candour, in closely guarded diaries and tapes, edited and annotated, even indexed, for posterity, whose verdict is clearly of huge importance. He described himself self-deprecatingly at a Dublin awards ceremony in 1977 as the “unknown gurrier of Irish letters”, but what he saw as the neglect clearly hurt.

J’s perspective on her father is that of a child of a first marriage, deserted at the age of eight, with her mother and younger brother, by her father. She was always somewhat distant, literally and emotionally, from an aloof, not-altogether-attentive but fond Denis. He left Dublin and his first marriage to become a BBC producer and then correspondent, and as daughter Ellie would complain in the novel, probably for J, “No one told me you had gone forever. No one gave me that piece of information for five years.”

Intra-family communication was never a Johnston strong point, and, later, although she would long as a writer for his support and approval, she recalls how it was only through a radio interview that she learned of his enormous pride in her literary accomplishment. Though in awe of and deeply attached to him there is also, one senses, a bit of what her fictional counterpart is alleged to feel about her father Desmond. “She considers you an old fraud,” he is told.

In 1938 Denis left Shelah, as fictional Desmond left Pamela, both men insisting that their marriage was broken, and, in fact, both before the break were already playing the field. Denis was involved with the successful younger actor Betty Chancellor, who would become his second wife, and, in a simultaneous on-off, torrid affair, with the woman he would later describe as the “real love of my life”, Nancy Horsbrugh-Porter. She would write to him that there were two schools of thought about him in Dublin: one that he was a gorgeous cad, the other that he was a cad. “And then there’s me.”

As luck would have it - well, luck had little to do with it - both women became pregnant almost simultaneously, giving birth only two weeks apart. But Denis had made his choice already in leaving Shelah for Betty, while Nancy, clear that Denis would not marry her, had agreed to marry another man. In a tape, Equinox, recorded at the age of 61 and found by the family after his death, Denis fondly recalls and explicitly describes the affair with Nancy, specifically two passionate encounters, one ahead of her marriage, the other only days after her honeymoon. No doubt about it, he could have been the father. “I had wanted another child, and now, by God, it looked as if I might be having two at the same time by two adorable girls, neither of whom was my wife.”

“She had a child, you know, not long after marrying,” Desmond says in Truth or Fiction. “They went to America almost immediately. Her cousin told me about the child. For years I did wonder on and off … As the years passed I realised it was probably not my child.” Denis was never sure and in his tape agonises about why she would not tell him.

As his seemingly interminable divorce wended its way through the courts, Denis escaped to the War in the desert of north Africa. “Dublin was too small for that to be comfortable for either of them,” Desmond says of his two lovers. “I was amused. I sat out in the desert and laughed when I thought of them … and of course I cried when I thought of how I had lost Abby [Nancy?]. You see the moment Anna [Betty?] told me she was pregnant I knew I would have lost Abby … I had to see Anna right.”

Denis admits ruefully in Equinox that “I really am a bit of a shit.” A cad, at least.

Truth or Fiction is set three decades later. Abby/Nancy’s marriage had lasted barely two years, and following a few years in the US she had returned to Dublin with her two boys. She would die young in 1952, largely alone, and in Denis’s papers we would find the title deeds and a photograph of her grave in Deansgrange. On it he had erected instead of a gravestone a stone birdbath inscribed with the words “Remember Nancy”.

In 1970, Denis would finally return from teaching in the US to Dublin with Betty and their two sons. J and her family would see him regularly, though there was always a mutual froideur between them and Betty, who never reconciled herself to his first family and their claims on him, emotional or other, a tension reflected in the novel. Both she and Shelah, who would move to nearby Sandycove, refused to acknowledge each other’s existence when they bumped into each other accidentally, although Shelah and Denis would meet surreptitiously for lunchtime assignations in a local pub, as do Desmond and Pamela.

When Desmond introduces her to a journalist anxious to “reopen” his life to a forgetful public, she relishes the chance. “I have plenty of secrets you know and if that’s what you’re ferreting about for I might just let you in on some of them.” As Desmond has second thoughts about the prospect, she brushes them off: “You said you wanted me to tell her the truth, what I want to know is it ‘your truth’ or ‘my truth’?” And then there’s J’s fictional truth. As the novel’s journalist says on hearing the tape: “I don’t know what this is all about. It’s fiction, isn’t it? Pretty ghastly fiction.” Riveting truth, though.