McMahon, 61, was tracked down by The Sunday Telegraph to his remote home near the Irish border just days before the 30th anniversary of Earl Mountbatten of Burma’s death.

The Queen’s cousin, who was a Second World War hero and the last Viceroy of the British Indian Empire, was assassinated by the Provisional IRA, who planted a bomb in his boat at Mullaghmore, Co Sligo, in the Irish Republic.

Along with Lord Mountbatten, 79, those to die in the explosion were Nicholas Knatchbull, 14, one of his twin grandsons, and Paul Maxwell, 15, a local boat boy. Another passenger on the boat, the Dowager Lady Brabourne, 82, died the day after the attack.

McMahon now lives with his wife Rose in a hillside bungalow in Lisanisk, close to the market town of Carrickmacross in Co Monaghan. His two sons are grown-up and he is believed to work as a carpenter from his home, which is decorated with some of the landscapes he painted during his 18 years in jail. When approached by this newspaper on Friday, he initially denied being McMahon. Then he added: “Take yourself off.”

Those who struggle to forgive McMahon for his actions include John Maxwell, the father of Paul, the dead boat boy, and Timothy Knatchbull, whose twin brother died in the blast. In less than a month’s time Mr Knatchbull, who was himself seriously injured by the explosion, will publish his book on the events of 30 years ago.

John Maxwell concedes that McMahon has served his time, but says the IRA had known that his son would be on the boat but had decided that his innocent life should be sacrificed nevertheless.

This weekend Mr Maxwell said of the killer: “I would like some kind of explanation from him in relation to his actions.” Furthermore, he revealed that his son’s killer has snubbed two requests for a meeting.

“I’ve made two approaches to McMahon, the first through a priest, who warned me in advance that he thought there wouldn’t be any positive response. And there wasn’t. I have some reservations about meeting him, obviously – it might work out in such a way that I would regret having made the contact. On the other hand, if we met and I could even begin to understand his motivation. If we could meet on some kind of a human level, a man to man level, it could help me come to terms with it. But that might be very optimistic. McMahon knows the door is open at this end.

“There is one question above all others that he would like to ask McMahon: “I think I would put a question to him directly, along the lines of...’If I had killed his son, for whatever reason, how would he feel about it? Would he be capable of putting himself in my shoes, to look at it from my angle?’ I’d be interested to know how he would reply to that.”

The terrorist attack three decades ago led to one of the biggest police investigations in Irish history. Two men were charged: McMahon, then 31, and Francis McGirl, 24, a gravedigger.

At the time of the explosion, McMahon was 70 miles away, in police custody – by chance he and McGirl had been stopped at a checkpoint after he had laid the explosive. One of the IRA’s most experienced bomb-makers, McMahon had flakes of green paint from Lord Mountbatten’s boat and traces of nitroglycerine on his clothes. The bomb had been detonated by remote control at 11.39am when the boat, Shadow V, was about 200 yards from the harbour.

Because there was insufficient evidence to place McGirl at the fishing village of Mullaghmore, he was acquitted and he died in 1995. McMahon was released from jail in August 1998 as part of the Good Friday peace agreement.

McMahon has never publicly discussed his role in the bombing. However, the year before his release from jail, his wife said: “Tommy never talks about Mountbatten, only the boys who died. He does have genuine remorse. Oh God yes.” Mrs McMahon has been politically active: she is a former Sinn Fein councillor and a former Mayor of Carrickmacross.

McMahon had served the first 13 years of his life sentence in the IRA wing of Portlaoise. He and ten others, armed with guns and explosives, failed in an attempt to escape in 1985. Three years later, he fired a shot from a Browning pistol smuggled into a holding cell of Dublin’s Four Courts. But, in 1992, he claimed to have turned his back on the IRA.

Three people on the boat survived the attack on Mountbatten including Timothy Knatchbull, 14, – Nicholas’s twin brother – who was left with serious injuries after the blast. At Lord Mountbatten’s memorial service in December 1979, the Prince of Wales lashed out at “the kind of subhuman extremist that blows people up when he feels like it.” Prince Charles had always been close to his great uncle.

Timothy Knatchbull’s book, From A Clear Blue Sky, is being published to coincide with the 30th anniversary of the bombing on August 27, 1979.

Caroline Gascoigne, the publishing director of Hutchinson, which has bought the rights to the book, said: “Tim’s motivation for writing it was a personal journey of healing and reconciliation, and also of finding out what happened. Part of the book is an anatomy of an assassination and it’s almost like a detective story as he assembles who was doing what and where.”

The new September issues of Tatler magazine reveals that Timothy Knatchbull, who was blinded in one eye by the blast, found solace after his brother’s death by forging a friendship with a man who, like him, had suffered the loss of his identical twin brother.