One man has drowned and two others swam to safety when their car plunged in to the sea.
The accident happened at Courtown Harbour in Co Wexford at 10.30pm on Friday night.
Gardai said the car hit a wall before going into the water.
Two teenagers managed to swim ashore and the third, aged 19, was rescued from the water by coastguard and emergency services.
"He was pronounced dead at the scene," a Garda spokesman said.
"His body was taken to Waterford Regional Hospital for a post mortem examination."
Investigators have appealed for witnesses.
Elsewhere a 21-year-old man died when his car hit a grass verge at Clooneshil near Newport, Co Mayo, shortly after 3am on Saturday morning.
A woman in the car was also injured and taken to Mayo General Hospital.