A sea and air rescue mission was mounted after reports that the car had fallen into the water at Buncrana, a popular tourist town on the bank of sea inlet Lough Swilly.
Five bodies, reportedly including three children, were recovered from the car. A young child was rescued and taken to hospital.
"We got an emergency call of a car that was off the pier at Buncrana," said a spokesman for the Irish coast guard at their nearby base in Malin Head yesterday.
Martin McGuinness, deputy first minister of Northern Ireland, posted on Twitter: "Very sad news of fatalities in an incident at Buncrana Pier tonight. Thoughts are with relatives of those involved and rescue services."
Police said the cause of the accident was not yet clear pending an investigation.