Jeremy Beshoff, owner of a car dealership, had accompanied a 45-year-old man on a test drive of the car when "before I knew it he had driven off the pier".
"I tried to open the door but I couldn't because of the pressure against it from the water. I managed to smash the window and get out. I am lucky to be alive. I was in shock after it happened but I'm alright today, I'm recovering."
Mr Beshoff offered his sympathies to the drowned man's family. "It was horrific for them and for anyone else who knows him."
Mr Beshoff had no hint of what was about to unfold when he accompanied the man, believed to be from Coolock, Dublin, on a test drive of the Mini Cooper car. The car was apparently driven deliberately off the pier with the two men inside.
The 2005-registered red car was advertised online for €13,950 and described as having 16-inch alloys, two-tone red and black sports seats and a one-year warranty.
It was the first time the young businessman had ever met the potential client and Mr Beshoff said: "He seemed like a harmless type of person."
The man had made a number of previous phone calls to Beshoff Motors and visited the premises on Tuesday to take the Mini Cooper for a test drive.
"At 3pm he came in to see the car and he was in no way threatening," added Mr Beshoff, whose father, businessman Gerard Beshoff, is a member of the well-known Beshoff fishmonger family.
Jeremy Beshoff climbed into the passenger seat of the car and they drove into Howth village about 15 minutes later. The car came to a stop near the edge of the middle pier.
It then appeared that the man deliberately drove the car over the edge of the pier and plunged it 20ft into the water below.
Onlooker Martin O'Keeffe alerted the emergency services. He described how the car "was parked about 14-18 inches away from the edge of the pier". A few minutes later the car appeared to reverse and go forwards a few times before driving over the edge.
An RNLI lifeboat, Dublin Fire Brigade and the Howth Coast Guard mounted a rescue operation.
The driver's body was recovered. He was taken to Beaumont Hospital where he was pronounced dead.