The incident happened off Wexford's Hook Head this evening.
The father of four in his forties was taking part in an underwater search for a fisherman who went missing from a trawler off the Saltee Islands yesterday morning.
The alert was raised shortly after noon today as he returned to the surface after a dive.
He was treated by colleagues at the scene and airlifted to University Hospital Waterford where he has since been pronounced dead.
Wicklow Hails Role Model Warren Kavanagh Who Calls Time On Successful Career
Wicklow Councillors Split On Local Property Tax Rates
Wicklow's Jack Reynor To Be Honoured In Los Angeles
Ominous Soundtrack Drowns Out Councillor's Message As Row Over Council Rent Hikes Rumbles On
East Coast FM's Jack Massey Among Ten Selected for the Learning Waves Journalism Graduate Programme