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.
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Noon - 4:00pm
East Coast Afternoons
Light rain
High: 11°C | Low: 7°C
Light rain
High: 11°C | Low: 7°C
Light rain
High: 11°C | Low: 5°C
Light rain
High: 11°C | Low: 5°C
Light rain
High: 12°C | Low: 6°C