The thunder of an explosion tore that perfect blue sky on that day in Mullaghmore 40 years ago next week (August 27) and shattered the lives of many, not least the Maxwell family from Fermanagh.

15-year-old Paul Maxwell died after a IRA bomb ripped through a boat off the coast of Mullaghmore killing him along with the Queen’s cousin, Lord Louis Mountbatten and his 14-year-old grandson Nicholas Knatchbull.

The bomb on board the Mountbatten family’s old, green fishing boat, Shadow V exploded as it made its last lobster-pot excursion of the summer holidays in the picturesque County Sligo bay.

The Dowager Lady Brabourne died the day after the attack and Nicholas' twin brother, Timmy, his father and mother Patricia and John Brabourne were seriously injured.

The boat trip was a regular holiday outing for Mountbatten and his family who spent time at their castle, Classiebawn, in Sligo, every summer.

But it ended in such tragedy.

Hours later, two more bombs went off 100 miles away, killing 18 soldiers from the parachute regiment, the biggest single loss of the British army in Northern Ireland.

On Monday night a BBC documentary The Day Mountbatten Died showed how that loss from four decades ago still hurts very many.

A memorial to Paul exists in Enniskillen four decades later in the shape of Enniskillen Integrated Primary School, a tribute to the schoolboy whose father believes educating children from different backgrounds under the same roof can defuse the sectarianism which still exists throughout Northern Ireland.

Motivated by his own personal tragedy and spurred on by another IRA bomb which murdered 11 people at the Cenotaph in Enniskillen on Remembrance Sunday 1987, John became one of the driving forces behind integrated education in Fermanagh.

That, he believes, is Paul’s legacy and next week he will quietly remember his son privately 40 years on from one of the darkest days of the Troubles.