'The bombers were prepared to sacrifice children's lives ... it was a war crime'
In 1975, Kieran Conway was gathering intelligence for the IRA. Speaking for the first time in public about Lord Mountbatten, he confirms: "In the mid-1970s, there was an operation cleared to kill (Lord) Mountbatten. He was to be ambushed either exiting or entering his castle.
"Well, it would have included four or five men. A spotter car somewhere distant, then the walkie-talkie communication between the people with the guns and the car to say he's on his way. You'd know roughly how long it was going to take and then (you'd) open up on the car.
"The mindset in 1974, 1975, the early-70s, would have been operational, you know, like kill them, you know, without too much reflection."
In 1979, when the assassination took place, Anthony McIntyre was an IRA prisoner in the Maze. McIntyre subsequently fell out with the Sinn Fein leadership.
Looking back now, he is highly critical of the operation to kill Mountbatten: "Somebody knew children got on that boat ... not necessarily the guys who put the bomb on, or the guys who made the bomb ... the people who planned it certainly knew about the children and, if they knew about the children and were quite prepared to go ahead and to sacrifice their lives in order to get Mountbatten, then it's a war crime."
For Conway, there is no question that the responsibility for the operation ultimately lies with Martin McGuinness, widely believed to have been the IRA's chief of staff at the time: "That's the way it works. I mean, if you're the boss, you're the boss. You take responsibility for whatever goes on."
It was another 20 years before peace was finally established in Northern Ireland.
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In 2012, in a gesture of reconciliation, the Queen, second cousin to Lord Mountbatten, shook hands with McGuinness.
Irish Times reporter Olivia O'Leary says: "The symbolic strength of that shaking of hands was enormous."
A new documentary, The Day Mountbatten Died, commissioned jointly by BBC Northern Ireland and BBC Two, also tells the story of the IRA's ambush of a convoy of British soldiers later the same day at Warrenpoint, in which 18 were killed, including 16 members of the Parachute Regiment.
Anthony McIntyre recalls the mood among his fellow IRA prisoners on hearing of the news from Warrenpoint: "Exuberance, exhilaration - all our Christmases had come at once and come early."
For the surviving Paras, there is a grudging recognition for the sophistication of the IRA's military operation and of its significance.
As Private Graham Eve says: "The IRA was celebrating ... of course they were.
"You know, they had a good day. And doubly good in the sense, you know, with the memory for them of Bloody Sunday."
The film also tells the story of an overlooked victim of that day. Soldiers opened fire on Barry Hudson and his cousin, Bill, who had heard the explosions at Warrenpoint and wanted to see what had happened.
Twenty-eight-year-old Bill, who was on holiday from England, was shot dead.
The last word is reserved for India Hicks, granddaughter of Lord Mountbatten, who was 11 years old at the time.
She was watching a Laurel and Hardy film in the living room of Classiebawn Castle, the Mountbatten's family home in Mullaghmore, when she heard the explosion. She also lost her 14-year-old cousin, Nicholas Knatchbull.
She describes how the extended family is still struggling to come to terms with the tragic loss of that day. "Everybody coped very differently ... and some didn't cope well and, of course, we're seeing the side-effects of that, even to this day."
Sam Collyns is the director of The Day Mountbatten Died, which will be shown on BBC One Northern Ireland on Monday at 9pm