Observers with the Irish Whale and Dolphin Group believe this is just the second validated sighting ever - and the first time one has been photographed in Irish waters.
Amateur whale-watcher Ivan O'Kelly snapped the photos after hitching a ride with the 12.5m fishing vessel MV Atlantic Explorer (click here to view the gallery).
He was just hoping to spot fin whales, which are often found in the same areas as the tuna because, like blue whales, they feed on krill.
'(W)e didn't know at the time we were looking at a blue whale,' said Mr O'Kelly. 'It was colossal, like an oil tanker beside you. It was intimidating.'
Spotted some 60 miles into Atlantic fishing waters, Mr O'Kelly had to climb on the boat's fly bridge to photograph what he believed was a particularly big fin whale.
'It was certainly one-and-a-half times the size of our boat,' he noted. 'It had a speckled back, and was very fast moving, fast swimming and very muscular.'
The sighting of the elusive creature, which is so big its heart is the size of a car, is thought to have global significance for a species thought nearly extinct in the 1960s.
Padraig Whooley, IWDG sightings co-ordinator, who later confirmed it was a blue whale from the photographs is now trying to track its origin.
'This sighting will have not just Irish or EU significance - it is of global importance,' he said. 'This is only the second validated sighting, that we can stand over, of a blue whale in Irish waters this century.'
Mr Whooley has sent the images to an expert in Quebec, Canada, to see if the whale matches any others photographed in the North Atlantic.
A catalogue records 430 creatures, mostly from the Gulf of St Lawrence as well as some from Iceland, Greenland, Azores and Mauritania.
Software may narrow down possible matches before a pain-staking study of individual markings could positively identify the whale.
'It's a really, really important sighting and hopefully, it will mean the beginning of a return to Irish waters of blue whales,' Mr Whooley said.
Blue whales are believed to be the largest animals that have ever lived, far larger than even than the biggest dinosaurs.
They can grow to a length of more than 30m while weighing from 100 to 150 tonnes and are thought to live to be about 110 years old.
RELATED LINKS:
Call of a Blue Whale (captured in New Zealand by Mark McDonald)
