A man died after getting into difficulties in the sea at the Forty Foot yesterday.
A rescue operation by emergency services and the Coast Guard swung into operation around 2.30pm.
People in the water at Sandycove at the time also went to the man's aid.
Dun Laoghaire lifeboat, a Coast Guard unit and the Rescue 116 helicopter all assisted.
The Coast Guard took the man to Sandycove Point and he was taken by ambulance to St Vincent's Hospital. He was later pronounced dead.
Fianna Fail councillor for Dun Laoghaire Cormac Devlin said: "This type of incident is really unusual so it will be greeted with shock."
Fine Gael councillor for the area Lorraine Hall said: "This is a terrible tragedy and my heart goes out to the family and loved ones."