Lord Mountbatten's youngest grandchild has revealed how she was given Valium to cope with the murder of the royal in the Republic in 1979.
It was only by chance that India Hicks was not also on a boat blown up by the IRA, and instead decided to watch a Laurel and Hardy film at the family's Co Sligo retreat, Classiebawn Castle, when she heard the explosion.
In a BBC documentary to be broadcast on Monday to mark the 40th anniversary of the atrocity and the Narrow Water massacre on the same day, August 27, 1979, former model Hicks recalled how, then just a child, she was handed medication to cope.
Hicks - whose cousin Nicholas Knatchbull (14) was killed in the blast - recalled: "I think it's a reflection of the era that someone would have had Valium and said: 'Let's give it to the children'. I mean, dear God, would you give an 11-year-old a Valium?
"The damage that was done was so much deeper than any of us could ever have imagined and adult lives are still being horrifically disrupted."
She said the 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 we're still seeing the side-effects of that, even now," she said.
Veteran BBC journalist Nicholas Witchell told the documentary he believes the fact the Government did not bow to the IRA after the death of Lord Mountbatten on the same day as the Narrow Water massacre was a turning point in the Troubles.
He said the decision by then Prime Minister Margaret Thatcher to stand firm against the relentless terror threat forced republicans to realise they had to chose a democratic path instead.
Mr Witchell was a Belfast-based reporter at the time and was the first to confirm the murder of Mountbatten.
He was also told the royal had died alongside his grandson Nicholas and a local boy Paul Maxwell (15). The 83-year-old Dowager Lady Brabourne died the following day.
Later that day the IRA killed 18 soldiers at Narrow Water in Warrenpoint.
Mr Witchell (65) said: "Looking back, can it be claimed that these deaths marked a watershed? The answer, I suggest, is not in the way the IRA had intended.
"The killing certainly lifted the morale of hardline republicans: to some it was a propaganda victory. Sixteen of the dead soldiers belonged to the Parachute Regiment, the first battalion of which had been involved in the Bloody Sunday shootings.
"Yet the bombings undoubtedly deepened the resolve of the then Prime Minister Margaret Thatcher never to yield to the IRA." He added: "Two years later her Government faced down the republican hunger strikes. Ten members of the IRA/INLA were left to starve themselves to death at the Maze prison.
"It brought home to the more thoughtful members of the republican movement that there could never be a 'military victory'.
"If Britain could withstand those events of August 27, 1979 and, just 24 months later, stand up to the hunger strike, then might it be time to change course and embrace democracy? That, of course, is exactly what did happen with the Good Friday Agreement two decades later."
- The Day Mountbatten Died is on BBC1 at 9pm on Monday