An eyewitness watched a driver of a car scream for help as it slid into the sea off a pier, killing five people including at least two children.
Francis Crawford was with his wife in Buncrana in Co Donegal in the Irish Republic when the tragedy unfolded on Sunday evening.
He called 999 and pleaded with another man who was on the shore overlooking Lough Swilly to swim out to the car after the driver begged for help.
A baby was saved by the bystander and is said to be in a stable condition in hospital.
Emergency services said they recovered the bodies of two boys, a man and a woman and another female whose age has not been confirmed from the vehicle.
"The man was still shouting to me when the car went down. It was very sad," he said.
"There was nothing I could do or nothing anybody could do.
"Any of the people who could do something were coming as quick as they could and got here in great time and knew what to do.
"But it was all too late. There was nothing anybody could do. Everybody did what they could and it was too late."
It is understood gardai are treating the incident as an accident.
The car entered the water at around 7.30pm on Sunday evening.
The dead are all believed to be from Derry city.
Mr Crawford said the man who swam out to the car was a hero and risked his own life.
He stripped down to his underwear and swam to the Northern Ireland-registered sports utility type vehicle as it drifted off the slipway.
"How he got (the baby) ... they are saying it was handed out to him," Mr Crawford said.
"When he came back he was totally exhausted. I think if he had another five yards to go - I can't see how he would have made it."
A number of the people in the car are related.
Mr Crawford believed the vehicle had only just entered the water when he spotted it and shouted to the driver.
It is suspected the driver tried to reverse at the edge of the slipway but the wheels failed to grip on thick algae.
The pier is used for a local ferry service which runs across the Swilly from Buncrana to Rathmullan.
It is a very popular scenic spot, with hundreds of day-trippers visiting the area from Derry city and the north-west every weekend.
Mr Crawford described the panic that ensued with the car sinking within 10 minutes of it sliding down the slipway.
"All the time the car was bobbling about and I knew the services were all on their way and I was hoping against hope that the car would stay up until the people came," he said.
"I was watching the car and the nose of it started to dip and then all of a sudden the whole of it started to go down, nose first, and everything stopped."
A major air and sea rescue mission was mounted by the Irish Coast Guard, gardai and emergency services after the alarm was raised.
Two lifeboats, a helicopter and local fishing boats were involved in the efforts.
Mr Crawford broke down in tears recalling the final moments.
"It was harrowing and heart-rending," he said.
"I don't want to go into details.
"Feeling hopeless and helpless. Nothing you could do and the man (driver) was shouting to me and I said 'I've phoned them, everyone is on their way' and he was still shouting.
"You felt so hopeless and you watched it unfolding in front of your eyes.
"It was a surreal sort of thing."
Mr Crawford remained on the scene after the car sank and said he saw one body in the water between where the vehicle sank and the slipway.
Another could be seen floating further out from shore and a "smaller image" was visible out in the water.
"You couldn't just jump out of the car with children in it and leave them," he said.
Mr Crawford said emergency services brought bodies to the pier and tried to resuscitate them.
The bodies were taken to Letterkenny General Hospital, where post-mortem examinations will take place.
Northern Ireland Deputy First Minister Martin McGuinness said he was saddened by the news.
He said on Twitter: "Very sad news of fatalities in an incident at Buncrana Pier tonight. Thoughts are with relatives of those involved and rescue services."