A 33-year-old English man was found clinging to a rock northwest of Blasket Sound almost three hours after he attempted a solo swim between the Great Blasket and Beginish island off Co Kerry.
Valentia Coast Guard co-ordinated the rescue effort on Wednesday afternoon.
Pockets of heat haze or fog, which locals say is a seasonal feature around the Blasket, prevented the Shannon-based rescue helicopter from operating in the area.
The Coast Guard relied on information from a local trawler, ferries and the Valentia and Dingle Coast Guard boats in a three-hour search which began shortly after 3.30pm.
The man, a strong swimmer and believed to be from London, took the ferry from Dun Chaoin on the mainland to the Great Blasket accompanied by three friends.
Wearing a yellow cap and a wet suit, he set about the kilometre swim between the islands watched by friends.
However, sweeping currents caught him, and the thick haze soon prevented sight of his yellow cap.
He was eventually found at about 6pm north of Beginish by Dingle Coast Guard, having been swept in a huge circle by the current.
A trawler which passed through Blasket Sound earlier in the afternoon provided vital information on the currents. This helped locate the man, a Coast Guard spokesman said.