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 Councillors: Answers & Action Needed For Long Derelict Wicklow Courthouse
Three Wicklow Beaches Can Fly The Blue Flag This Summer
Local Radio Plays A Huge Role In Community Engagement
Improvements Required At Bray Disability Centre Following HIQA Inspection
Baltinglass Awaits Rás Stage Finish