Two others were located by the Howth RNLI in the Baldoyle estuary on Friday after reports of swimmers getting into difficulty.
The Coast Guard received a call at 1.35pm on Friday that two people had gotten into difficulty in the water at Cush Point.
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A search operation was launched after being told of a third teenager in the water, who was later found holding onto a buoy in the middle of the estuary.
The RNLI said he was tired but alive and that the operation could have had a tragic outcome.
During a separate call out on Saturday, the Howth RNLI responded to a speed boat which broke down north of Lambay Island, with a family of four on board.
A lifeboat responded to the call and towed the boat safely back to the Malahide Marina.
