An Irish Coastguard crew member who died after being pulled from the Atlantic has been named as Captain Dara Fitzpatrick.
The 45-year-old was earlier found in a critical condition as rescuers searched for a Coastguard helicopter that went missing in the early hours.
Capt Fitzpatrick had been with the search and rescue service for more than 20 years.
Search and operations manager Gerard O'Flynn told Sky News she was "hugely experienced" and "our most senior pilot".
Capt Fitzpatrick made Irish aviation history three years ago when she flew the first all-woman mission for the service.
An intense search for the three other crew is continuing off the west of Ireland, but officials say hopes are fading.
Contact was lost with the aircraft at around 1am while it was supporting another helicopter responding to a medical evacuation on a fishing boat 150 miles out to sea.
Helicopters from Sligo and Shannon, an Irish Air Corps plane and a Naval Service ship are involved in the search six miles off the County Mayo coast.
Five fishing boats, two lifeboats and police divers are also taking part.
Lifeboat operations manager in Achill, Tony McNamara, told TheJournal.ie that there was "quite a lot of debris" in the water.
Officials said the missing helicopter was called out to provide safety and communication support - known as "top cover" - for the rescue mission because it was so far out to sea.
The Sikorsky S92 aircraft had flown directly from Dublin and travelled across the country and 10 miles out to sea, before heading back towards land to refuel.
Its last communication was: "Shortly landing at Blacksod" - but it failed to arrive.
"Suddenly it became apparent that the aircraft had ditched 10 miles short of the fuelling stop - on the seaward side of the fuelling base," Mr O'Flynn told Sky News.
Visibility at the time was described as poor, but not dangerous.
Coastguard chief Eugene Clonan called it a "dark day for the emergency services".
Search and rescue expert David Mearns said the twin-engine helicopter involved is used all around the world.
"One had a near accident in the North Sea... It had to make an emergency landing... that was just in December," said Mr Mearns.
"With any of these helicopters they want to make sure the entire fleet isn't at risk. That will be a priority for the air accident investigation."