The man was swimming in the sea off Ardara, Co Donegal, when he got into difficulties at about 2pm on Saturday.
Gardai said the man was airlifted to Letterkenny hospital where he was pronounced dead.
Irish Water Safety has made repeated appeals for awareness of the dangers of swimming in areas without lifeguards, especially in open water, and the difficulty of swimming in cold water following a series of deaths in rivers, lakes and at sea.
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