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.
Tender Issued For DART Charging Infrastructure In Wicklow Station
Almost 1,200 Wicklow Children Waiting for Early Years Places
Wicklow Hosepipe Ban Extended As Supplies Remain Under Pressure
Wicklow Rents Rise Again As National Market Cools
UCD Study To Lay Bare The Blight Of 'Sheep Worrying' Across Wicklow