Scientists clocked a fin that looked 'unlike anything' they'd seen before in the Celtic Sea, around 100 miles south-west of Ireland.
They later determined it was a hammerhead shark - which have never been seen in the sea around Britain.
The creature was found during a survey of herring stocks, which was being carried out by scientists from the Marine Institute based in Galway, Ireland.
The survey is carried out every year on board the Marine Institute's RV Celtic Explorer, but this time experts spotted something unlike the sharks they'd seen before while exploring the area.
John Power, a marine mammal observer who spotted the hammerhead, said: "While scanning the ocean surface, we sighted a dorsal fin unlike anything we had encountered before. It was quite different to the fins seen on basking sharks and blue sharks.
"After consulting available ID keys, we agreed that the shark must be a smooth hammerhead."
The hammerhead (sphyrna zygaena in Latin) gets its name from its distinctive flat, hammer-shaped head. It refers to a group of sharks, including the smooth hammerhead species that was spotted in the Celtic Sea.
They may look a little ominous - as they can grow up to 20ft long - but experts say there have been no known fatalities anywhere in the world from hammerhead sharks.
Increasingly targeted for the shark fin trade, the smooth hammerhead species is listed as 'vulnerable' on the International Union for Conservation of Nature Red List of Threatened Species.
While hammerheads have not been seen in British waters before, Dr Simon Boxall, of the Southampton Oceanography Institute, believes there is no reason why they could not exist in these waters.
They're usually found in warm, tropical seas in areas like the Caribbean and West Africa, migrating to cooler waters in the summer.
There's a possibility that the lone shark spotted off the Irish coast was lost, or may have ventured further north than usual due to the waters getting warmer.
Boxall said: "I am not aware of them [hammerheads] being seen in our waters before, but this sighting does not surprise me.
"Temperatures in these waters have increased by 2.5C over the last 20 years and more exotic species carried by the Gulf Stream are travelling further north for food.
"But this also means that native fish species such as cod are also being pushed further north as they lose colder waters."
The sighting also comes a week after marine expert Dr Ken Collins, from the University of Southampton, said hammerheads could become a regular fixture to British waters by 2050.
Like Boxall, he believes hammerheads - and other shark species - could be attracted to the seas here due to 'rising temperatures'.
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