In the early hours of Easter Tuesday 1941, a line of fire brigades from the Republic quietly rolled across the border into the smoking ruins of a Belfast devastated by Nazi bombers.
The attack, which lasted four hours, had begun the previous night, April 15, when between 180 and 200 German warplanes relentlessly battered the defenceless city, leaving about 1,000 people dead, 10,000 injured and an estimated 100,000 homeless.
It's believed the objective was to disable two landmark industries -- the renowned Shorts factory, which manufactured planes, and the Harland & Wolff shipyard, which supplied warships to the British government.
The shock bombardment left parts of the city virtually unrecognisable and Belfast's emergency services completely over-stretched.
As dawn broke and realisation dawned of the full extent of the damage caused in the attack, an SOS for emergency services back-up was sent first to London and then to Dublin.
The British government quickly organised a team of 200 firemen to travel from Liverpool and Glasgow. However, as welcome as their support would be, it was recognised that it would be 24 hours by the time their cavalcade reached the city.
Belfast, it was clear, needed more immediate help.
Although there's a lack of documentation about what happened next, it's believed that John McDermott, the Minister of Public Security in Northern Ireland, sent an urgent message to the Irish Government. The message is believed to have been received by the city manager, who brought it to the attention of the then Taoiseach, Eamon de Valera.
"Mr de Valera decided to send fire brigades from Dublin, Dun Laoghaire, Drogheda and Dundalk; the exact number is not clear, but we believe that about 20 fire engines went, and about 71 firemen, along with a number of ambulances," says journalist Paddy O'Flaherty, whose grandfather was among the fire-fighters who travelled from Dundalk that night.
Before the Dublin firemen left for Belfast, they assembled in the yard at the Tara Street brigade headquarters in the city centre, says O'Flaherty, a freelance journalist who presents tonight's radio documentary on the Belfast blitz.
The documentary features retired fireman Tom Geraghty, who worked with some of those who went to Belfast and who wrote about the history of the Dublin fire brigade.
"He told me the fire chief spoke to the men from a metal balcony overlooking the yard -- like a general addressing his troops. He told them de Valera had asked for assistance for Belfast and there was no compulsion. He asked for volunteers and Tom said the men who went were proud they had volunteered.
"I'm especially proud of my granddad. I think it's sad we don't have a proper memorial to these men. They're like hidden heroes. We don't even know all of their names, but it's very important that we remember their bravery.
"They were going into the unknown," he continues. "Some of them had seen the destruction caused by heavy artillery during the 1916 Rising and the War of Independence, but none of them would have seen the horrific effects of aerial bombardment.
"I've been told that the authorities were terrified that one of them would be killed or injured in Belfast -- obviously the danger increased if they were caught up in another night raid so they were told to return to the Republic that evening," he adds.
O'Flaherty's grandfather, Patrick Rooney, was the driver on the Dundalk fire engine, which was the first of the Republic's aid contingent to enter the burning city.
There is no mention of this mission of mercy in the records of the Dublin fire brigade, partly because it was in breach of Ireland's neutrality and partly because of the fact that there were no regulations for compensating any of the volunteers in the case of death or injury. The initiative was a political hot potato.
"This is because people were afraid of repercussions from the Germans -- Ireland was a neutral country, so this was politically a very sensitive issue," O'Flaherty explains.
"Also, if any of the volunteers was injured or killed in Belfast or on the way there or back, there was no official structure for the provision of compensation -- the law did not allow for them to work outside specific areas so there was a risk being taken," explains O'Flaherty.
"There was some mention of the aid mission in the records in Dundalk, Drogheda and Dun Laoghaire, but as far as the official records of the Dublin fire brigade were concerned, nobody went to Belfast."
His grandfather's first-hand account of the dreadful events of April 15 and 16, which was handed down through the family, is one of the inspirations for O'Flaherty's 40-minute documentary. The other is the fact that this month marks the 70th anniversary of the blitz.
"My grandfather talked about it being a terrible night, the most dreadful night he'd ever experienced. He said the most horrific aspect of it were the bits of bodies that the firemen found.
"That preyed on his mind for a long time afterwards. It was a nightmarish scene -- it is one of these horrors that you'd have to experience to understand.
"These firemen were all volunteers. They took their lives in their hands," he adds.
A few weeks after the Easter Tuesday blitz, firemen from Dublin and Dundalk returned to Belfast following another raid on May 4, which left 191 people dead.
Dublin also felt the fury of the Nazi war machine later that month. At 1.30am on Saturday, May 31, the Luftwaffe attacked the North Strand area. Thirty-four people were killed, 90 were injured and nearly 300 houses were destroyed.
Some people believe this was a German reprisal for sending help to Belfast.
"The fact that the firemen came from the south to the aid of Belfast is part of my family history, so I was very surprised to discover that nobody knew about it. That reason, along with the fact that this year is the 70th anniversary of the blitz, is why I decided to do the piece," says O'Flaherty.
Up to April 1941, nobody ever expected Belfast to be targeted by the German air force, primarily because it was believed the city was beyond the range of their bombers.
But Belfast was a tactically important location, and the Nazis soon solved the distance problem.
"Belfast was targeted because it was an important industrial centre. Several years earlier, the Shorts factory was moved from its headquarters in the southeast of England to Belfast amid growing concern that its reputation as a hugely significant supplier of planes would make it a target.
"It moved to Belfast around 1937 because it was feared that its then headquarters in Rochester would be vulnerable to attack by German bombers if war broke out -- as it did in 1939.
"Another prime target was the Harland & Wolff shipyard, which was still the world's leading shipyard, and it was a very important supplier of warships for the British government. Both were extremely badly damaged in the blitz of April 16," says O'Flaherty.
"It was thought that Belfast was beyond the range of German bombers, that they wouldn't have been able to carry enough fuel to get them that far."
However, all that changed when France was occupied. Now, Germany had access to airfields in the north of France, making the journey to Belfast much shorter.
"It was a very easy journey from the Brest Peninsula in north-western France across the St George's Channel over Cornwell and straight up the Irish Sea."
After the war, aerial reconnaissance photographs were recovered which showed these two factories had been clearly marked for the bomber pilots.
"They knew exactly what they were aiming for. The blitz happened in the middle of the night," says O'Flaherty, who adds that this wasn't the first time Belfast was bombed -- the Germans had initially attacked the bustling city on April 7.
"Belfast was bombed for the first time on April 7 and the shipyard was badly damaged on that occasion."
However, the raid of April 7 clearly demonstrated to the Nazi war strategists that Belfast was virtually unprotected.
It was, they realised, practically a sitting duck for their next, more serious assault on April 15/16 -- and which, though followed by the bombardment of early May, was by far the worst of the three.
In that four-hour period between the night of April 15 and the early hours of April 16, German fighter planes dropped more than 200 tonnes of bombs on Belfast. Much of the city centre and the northern areas were devastated. Stretches of well-known streets such as the Antrim Road, York Street, the New Lodge Road and Duncairn Gardens were destroyed.
"The city was brought to its knees, partly because there were no defences in place," says O'Flaherty.
Retired Belfast GP Dr Seán Gibson, who was 14 at the time of the blitz, is one of those who vividly recalls the sheer horror of that night. "He lived on the Antrim Road and remembers it very clearly. He and his family hid under the table during the bombardment. The damage to the area was so bad that they discovered their four-storey mid-terrace home was now the end house -- the three houses beside it had been completely wiped out," O'Flaherty explains.
Another eyewitness was Rita Brown, now aged 91, who was attending a concert by renowned singer Delia Murphy in the Ulster Hall in Belfast City Centre that night.
Because it was so dangerous out on the streets -- bombs were dropping all around the city centre -- most of the audience was forced to remain in the hall.
The courageous Murphy continued to sing as bombs peppered the city, and somehow kept the terrified people relatively calm as their beloved city buckled under the force of the explosives raining down upon it.
The terrified audience could clearly hear the bombs exploding outside, says O'Flaherty, but Murphy kept going, somehow managing to prevent them fleeing the hall. A few hours later, when O'Flaherty's grandfather drove into the city, he and the crew were met by a panorama of devastation which many of them never forgot.
Now, 70 years later, the Dundalk fire brigade still has the fire engine that Patrick Rooney carefully steered through the rubble and the ruins that night.
"It was purchased in 1936 by Dundalk Council. It has been completely restored and it's in excellent running order. It was even in the recent St Patrick's Day parade -- and now I'm hoping to go for a drive in it myself!"
'Documentary on One: The Hidden Heroes of the Belfast Blitz', tonight at 6.05pm on RTE One. Repeated tomorrow on RTÉ Radio 1. See www.rte.ie/doconone