Veteran rock music writer Stephen Davis, who has known Simon for years, claims in a major new book that the hippy-chick New York singer had a passionate affair with the public school-educated thespian when he was already long married to Irish actress Sinead Cusack, his wife of nearly 34 years.
According to Davis, Simon's affair with Irons began in the summer of 1984 when she hired the actor, then 36, to direct a pop video for her song Tired Of Being Blonde.
He had no directing experience in anything, let alone a pop video, but, according to Davis, Simon may have had an ulterior motive. She'd had a 'long-time crush' on him, dating back to his role as Charles Ryder in the 1981 TV adaptation of Evelyn Waugh's novel Brideshead Revisited.
That summer of 1984, Irons arrived at Carly Simon's home in Martha's Vineyard — the millionaires' summer paradise near Boston, where she still lives — with his wife Sinead Cusack and their young son, Sam.
Simon had two children of her own by her first husband, the singer-songwriter James Taylor, but had recently ended her marriage to the musician, who had a serious heroin and cocaine problem at the time.
The Irons family stayed with Simon for some weeks, spending their days on the island's white sandy beaches or being 'treated like royalty' by their gracious hostess and her staff.
The singer and the actor worked on ideas for the video, the plan being that they would shoot it when Irons returned to the island in September — sans famille.
He stayed in another of Simon's homes on the island, 15 minutes' drive away in the pretty fishing village of Menemsha, and they were soon more than artistic partners.
Whatever the extent of their relationship, Irons — who declined to comment when the Mail approached him this week — and Simon managed to keep it quiet.
Apart from a New York gossip columnist reporting that the pair had been spotted 'deep in conversation' over glasses of tea at New York's Russian Tea Room in 1986, they have rarely been mentioned in the same breath.
That said, Irons's relationship with Cusack has long been regarded by observers as an open one. Irons has described their marriage as 'dysfunctional', which, given his reputation as a man who has been caught in clinches with much younger women, may not be entirely surprising.