Ion Paun (23) had been confronted for trespassing at a service centre that he had been barred from.
Blanchardstown District Court heard the garage owner had been intimidated by a "ball of guff" that Paun had given him over the seizure of a car that had been abandoned in his car wash.
He was fined €650 for public order offences.
Paun, of Riverforest, Leixlip, had denied trespassing in a manner likely to cause fear at Lohunda Autocare, Clonsilla, on July 28. He had admitted a charge of breaching the peace.
Detective Garda Mark Ferris told the court Brendan O'Brien, the service yard owner, called him to the premises on July 28. He pointed out the accused who had made threats to him and had previously been barred from the premises.
The reason was that a vehicle had been abandoned in the car wash by Paun a week earlier, causing "substantial loss" to Mr O'Brien, and had been seized.
Garda Ferris said that while speaking to Paun, the accused "grabbed his genital area and told me to suck his d***".
Mr O'Brien told the court the accused had been with a group of people who "were intimidating me and my staff in the garage and we barred them from the garage because they had abandoned a car in the car wash".
He said Paun came and asked where the car was and was told it had been seized by gardai.
"They gave a ball of guff trying to intimidate us," he said. "They were shouting, bawling like they normally do".
Previously, they had threatened to "do harm to the garage".
Paun said in evidence he had come to visit his uncle who lived nearby and parked outside the garage. He alleged Mr O'Brien and his brother closed the garage gate and went to punch his friend.
His friend ran away and Mr O'Brien came and knocked on the car window, he alleged.
"I said, please, my child is in the front, what is wrong with you?"
Paun told the court: "He left me alone and then the police came. I did nothing, it wasn't my car."
He insisted he had not spoken to Mr O'Brien, and had not been barred.
Judge Hugh O'Donnell said he accepted Mr O'Brien's evidence. Paun accepted his behaviour had been "appalling", his solicitor Kelly Breen said.