On August 27th, 1979, the IRA also killed 18 British soldiers in two huge blasts in Warrenpoint, Co Down. Northern Editor Gerry Moriarty, one of the first reporters there, recalls the scene and aftermath.
Twenty-five years ago this morning the late managing director of the Donegal Democrat, Cecil King, came to the door of our newsroom in Ballyshannon, Co Donegal where I was a young reporter.
"I just got a call from the IRA," he said. "They said they've just blown up Lord Mountbatten's boat - and Lord Mountbatten is on it."
A few minutes later we were speeding up the nine miles of road that brought us past Classiebawn Castle, where Lord Mountbatten spent his summers, and into the village of Mullaghmore, Co Sligo, parking just close to the little harbour.
It was a strange scene. There were people out sunbathing and swimming on the beach - it was a blistering hot day - perhaps unaware that Lord Mountbatten's Shadow V boat had just been blasted to matchwood by a remote control IRA bomb.
At the harbour the dead and injured were being stretchered ashore from little boats that had immediately gone to the scene of the explosion.
Ambulances were in the process of ferrying the bloodied, battered and bewildered survivors to hospital in Sligo and the dead to the morgue in the town. At that stage it was known that 79-year-old Lord Mountbatten was dead. And so was his 14-year-old grandson Nicholas Knatchbull. The teenager's parents, Lord Brabourne and Countess Mountbatten - Mountbatten's daughter - survived, as did Nicholas's identical twin brother Timothy. But Lord Brabourne's mother, the Dowager Lady Brabourne, aged 82, died 24 hours later from her injuries.
From the vantage point where they detonated the bomb the IRA members could see they were killing children and other innocents that day (insofar as anyone could view Lord Mountbatten as a so-called legitimate target). But it didn't stop them.
There was still one boat to come ashore, a little half-decker making its way in the glimmering sunshine to the harbour wall. And there on the wall I noticed a bronzed, bearded, muscular man wearing only shorts and sandals pacing up and down, beside himself with grief.
It was John Maxwell (then 43) from Enniskillen, Co Fermanagh, father of the fourth fatality of that terrible day, 15-year-old Paul Maxwell, who while holidaying in Mullaghmore made friends with the Mountbatten party and was invited on the boat trip that day, August, 27th, 1979.
As the boat berthed beneath the harbour wall John Maxwell, a Protestant of unionist background - but who then as now despairs of nationalisms, whether unionist nationalism or Irish nationalism - looked down on the dead body of his beloved son and cried repeatedly: "Look what you've done to him, look what you've done to him. I'm an Irishman, he's an Irishman, is this the sort of Ireland you want?"
It's a phrase that bore deep down into my own consciousness and the few others of us who looked on helplessly that day.
On the same day the IRA in two huge blasts killed 18 British soldiers in Narrow Water in Warrenpoint, Co Down.
The focus in the British papers unsurprisingly was on the death of Lord Mountbatten, his family and their British royal family connections, and on the soldiers' deaths.
The tabloids in particular and some unionist and British politicians screamed for vengeance, branding every Irish person as culpable, and complicit in the slaughter.
John Maxwell took a different tack. He was then a senior sociology lecturer and a physical education instructor in Fermanagh College of Further Education in Enniskillen, also working with the Open University.
In a letter to the local Impartial Reporter he responded to some of the blood-curdling commentary in other papers by begging for no retribution and saying simply that it wasn't the Irish people who killed his son, but the IRA.
He said whoever organised and executed the attack did so of their own free will, yet it was too simple to brand the IRA as "sub-human", as was the general flavour of coverage at the time. There were other complex factors that could motivate such a heinous crime, he said.
I remember interviewing John Maxwell five years later for the Irish Press, where I then worked, and where he expanded on that enormous question he posed in Mullaghmore - is this the sort of Ireland you want? "I think Catholics and Protestants have more in common with each other than with England," he said.
"Remember my people are here 400 years. I am Irish but it is a different form of Irishness, and I can resolve that sentiment with holding a British passport," he said in a remark that in its own way encompasses the unionist-nationalist accommodating philosophy of the Belfast Agreement.
At the time John Maxwell expressed his willingness to speak to Thomas McMahon from Carrickmacross, Co Monaghan, the only person convicted of the crime, who served almost 20 years in prison and was freed in 1998. "Would Thomas McMahon accept that Paul was an Irishman?" he asked.
"I don't hate Thomas McMahon but I hate what he did. If he had known me, would he have done the same thing? Can he imagine what it's like to lose a son?" John Maxwell made two attempts through an intermediary to speak directly to Thomas McMahon but his overtures were rebuffed.
Chatting to John again 25 years on there is still a hugely impressive strength and gentleness about the Enniskillen man. He would still like to speak to Thomas McMahon, not to lecture, or to berate, but to seek to understand.
"I know there would be great psychological risks for me and my family, but I would be curious to find out what kind of man Thomas McMahon is, what motivated him, what he feels now about what he did, especially as I understand he is a father of sons himself," he said a few days ago.
"I've been reading a lot about truth and reconciliation and know that it does not work for everybody. I am not naive but maybe if there was some kind of shared common humanity between us it would probably help me - it might even help Thomas McMahon."