More than 2m people have watched the CCTV images of the attacker unsuccessfully throwing a stone at the car window outside a pub.
When he fails to smash the screen, he walks off before returning with what appears to be a brick, which he launches at the luxury coupé.
But in a split second, the makeshift missile rebounds, striking him in the head, and knocking him to the ground.
Gerry Brady, owner of the car and The Pheasant pub in Drogheda, Co Louth, said he found the man lying “completely dazed” beside the Mercedes after he finished work.
“He was lying on the road. I asked him: ‘Are you OK mate?’ as you do,” he said.
“There was a lot of blood. He couldn’t stand up. He was completely dazed. But he started saying his friend jumped on him and beat him up. My girlfriend was with me and she phoned the guards and an ambulance.
“When I told him the guards were on the way, he was saying to me ‘no guards, no police’, so I knew something was up.
“Then I looked at my car and there were some divots taken out of the window and a concrete block on the ground. The penny started to drop at this stage.”
Mr Brady said his girlfriend checked her own car nearby and it had been broken into, with an iPad and phone missing. The items were later recovered.
The pair ran to a nearby filling station where they hid until the emergency services arrived on the scene.
Mr Brady said he took his pub’s CCTV footage to the local Garda station and made a statement.
He has since put the coverage on YouTube, where it has been viewed more than 2m times.
The gardaí said they are investigating the incident.