JP Donleavy's portrayal of the adulterous, bed-hopping exploits of a student in post-war Dublin earned him fame and notoriety in equal measure when his debut novel, 'The Ginger Man', was published.
But while the American-born author has happily courted controversy through the sexually charged pages of his books, an extraordinary story of infidelity in his own life has remained a closely guarded secret for more than 30 years.
It has emerged that neither of the children born during his second marriage, to the actress Mary Wilson Price, was fathered by him.
The former Mrs Donleavy had affairs with Kieran Guinness and his brother Finn, scions of the brewing dynasty, and had a child by each of them while she was still married to Donleavy.
Kieran Guinness is the father of Rebecca (32), while Finn Guinness is the father of Rory (30). And although the novelist has known for more than 20 years that he is not their father, he has never admitted it publicly, and lists them in his entry in 'Who's Who'.
Kieran Guinness (62), who breeds racehorses in Ireland, said he remains a close friend of Donleavy, who lives on a 180-acre estate in Mullingar, Co Westmeath, but still has no idea when the author first discovered the truth.
He said: "I've no means of knowing whether he knew he wasn't their father when they were born," said Mr Guinness, who met the former Mrs Donleavy when he was the master of the local hunt in the 1970s.
"I've sometimes wondered. I don't consider any of it to be a secret, but obviously one doesn't want to upset anyone. He is a friend and a close friend and I'm fond of him."
According to one report yesterday, DNA tests were carried out when Donleavy and his wife divorced in 1988 that confirmed the children were not his.
Mr Guinness added: "There was talk of a DNA test, but I don't know quite what happened -- it was 30 years ago."
After her affair with Kieran Guinness, the then Mary Donleavy fell in love with his older brother Finn and married him in 1989. She changed her children's surnames to Guinness after they moved to Wiltshire, where they were brought up on Biddesden Stud.
Rebecca, now a fashion writer, and her half-brother Rory, a musician, were later told about their true parentage but have never spoken about it publicly.
Rebecca Guinness said in one recent interview: "When we moved to England, we became totally different people. We changed our names, changed everything. Our parents are farmers, you know, humble beginnings. They breed event horses in Wiltshire."
Mrs Guinness, meanwhile, remains friends with her former husband and went for tea last week at the former marital home, Levington Park, where Donleavy (85) lives as a virtual recluse with Philip, one of two children from his first marriage to Valerie Heron, whom he divorced in 1969.
He rarely gives interviews, but once described how Mary would "organise these parties down by the lake with bonfires, dancing, music and a pig roasting on a spit" while he stayed indoors, preferring his own company.
He has admitted in the past that Sebastian Dangerfield, the anti-hero of 'The Ginger Man', was partly based on his own experiences when he studied at Trinity College after serving in the US Navy during World War Two.
The book was initially banned in Ireland and the US, but went on to sell 45 million copies worldwide.
Mary Guinness said: "JP has known about it for a very long time, but it is a very touchy subject. All I will say is that my children are very, very lucky because they have the loveliest people involved in their lives in a paternal way -- Finn, Kieran and JP -- who are loving and supportive to them equally."