On 15 April, 1941, some 200 tonnes of bombs were dropped on Belfast, leaving 1,000 people dead and thousands homeless.
Volunteer crews travelled from Dublin, Dún Laoghaire, Drogheda and Dundalk and they found much of the city destroyed after the four-hour bombing raid.
Even though Ireland remained neutral during the war, Taoiseach Éamon de Valera gave the firemen permission to help.
70 years later, De Valera's grandson Éamon Ó Cuív is calling for a memorial to those Irish firemen who dashed north to help Belfast during WWII.
Speaking in RTÉ Radio One's 'Hidden Heroes' documentary, Éamon Ó Cuív said his grandfather was very conscious that he was taking a risk in allowing the firemen to cross the border.
The Germans could have taken the view that it was a violation of neutrality, he explained.
'But he weighed up the risk and decided very, very quickly that here were people on the island of Ireland, Irish people, who needed support, who needed help ... and that there was no way could he or should he refuse and that he should take the risk and authorise the fire engines to go to Belfast.'
Mr Ó Cuív also praised the courage of the Irishmen men who volunteered and has called for a joint north-south commemoration to honour the firemen's bravery.
He said there should possibly be a permanent memorial 'commemorating what was, in a difficult time, one of the important north-south gestures.'
However, the documentary reveals that there is no record of all the firemen's names and that the Dublin Fire Brigade chronicles omit any reference to their mercy mission.
Scant details were recorded by the fire brigades in Dún Laoghaire, Drogheda and Dundalk, it reveals.
Mr Ó Cuív said he wasn't surprised by the lack of information.
'That would minimise the risk of anybody arguing that there had been a pre-planned breaking of neutrality. I am sure part of the reason was to mollify any question the Germans might raise about it.'
The request for help after the attack by the Luftwaffe bombers was made by Stormont Minister for Public Security John McDermott.
He asked Deputy Prime Minister Basil Brooke for permission to request assistance from Dublin, who agreed.
The message reached de Valera possibly via the railway telegraph because telephone lines had been destroyed.
The story of the Irish firemen was not officially recorded, partly because of fear of repercussions from Germany, but the documentary tells the full tale on the 70th anniversary of the event.
Outside of London, the death toll from the night was the greatest loss of life in a single night air raid during the war.
The main targets were the Harland & Wolff shipyard, the Short Brothers aircraft factory and James Mackie's engineering factory.
With few air raid shelters in the city, the documentary tells how some people, Catholics and Protestants, took refuge in the crypt at the Redemptorist Monastery at Clonard in West Belfast.
Blitz survivor Rita Brown reveals how singer Delia Murphy continued an Ulster Hall concert while Luftwaffe aircraft dropped tonnes of bombs on the city.
The documentary also includes interviews with relatives of some of the firemen who went to Belfast.
Read more about the making of the documentary
Listen to Hidden Heroes of the Belfast Blitz