A mechanic claimed he was provoked and he "loafed" another man in a pub assault following a long-standing monetary dispute.
The victim had claimed that Robert Hughes (57) had headbutted and punched him during an attack that left him with red marks to the face.
There was still "bad blood" between the men, a court heard.
Judge Dermot Dempsey convicted and fined Hughes €300.
The defendant, with an address at Oberstown in Lusk, admitted assaulting another man at Martin's Bar, Old Road in Rush, north Dublin on January 27, 2019.
Garda Karl McManus told Swords District Court that Hughes headbutted and punched the defendant, leaving him with red marks to the face.
The victim had also suffered flashes in his eyes and had gone to see an ophthalmologist in August 2019, Gda McManus said.
Defence solicitor John Hennessy said that this was a pre-existing issue and was not caused by this assault.
Gda McManus accepted that the victim did not suffer any long-term effects.
He said Hughes gave a statement to gardaí where he accepted he had "loafed" the victim, though the victim claimed it was more than one punch.
The two men were known to each other, the garda said, and had previously worked together.
Gda McManus also told the court the victim did not wish to make a victim impact statement.
Mr Hennessy said there had been "an element of provocation immediately prior to the offence", and this had been confirmed by a witness.
He said there had been a long-standing monetary dispute between the two men.
The solicitor said Hughes wished to apologise to the court and he accepted his actions were contrary to the law.
However, Mr Hennessy added that he had not been instructed to offer an apology or compensation to the victim and that "bad blood" remained.
The court heard Hughes was a self-employed mechanic, was marred and had three children.
Fining Hughes, Judge Dempsey told him it was "time he got some sense".