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.
Former US Marine Jailed After Grooming and Abusing Wicklow Teen
Wicklow Property Prices Rise by €7,500 In One Year
Peter Thiel Event Will Not Go Ahead In Wicklow Hotel
Wicklow Receives €10.72 Million to Repair Roads Damaged by Storm Chandra
Wicklow Apprentice Wins ESB HGV Mechanic Apprentice of the Year