A young man punched another male in the face when a fight broke out at a Luas stop in the city centre.
A court heard there was a "lot of blood" and an ambulance was called.
A judge told Sean Jennings (23) he would consider leaving him without an assault conviction if he paid €200 compensation.
Jennings, of no fixed address, pleaded guilty to assault and threatening, abusive and insulting behaviour.
Dublin District Court heard gardai were called to a public order disturbance at the Connolly Luas stop on Amiens Street at 7.50pm on a date last July.
The accused got into a fist fight with the victim, punching him once in the face and causing bruising.
He continued to act aggressively when gardai investigated and was arrested.
Defence solicitor Kate McGhee said a summary of the evidence stated there was "a lot of blood" and it looked bad.
An ambulance was called, but there was no injury and "it turned out he was fine".
The accused had been co-operative.
A fight had broken out among a group of young people and the victim punched one of Mr Jennings' friends first, Ms McGhee said.
Jennings was not arguing self-defence, but "it was a group of young boys fighting".
Jennings also admitted possession of heroin after gardai found €30 worth of the drug on him in the north city centre last April.
On a later date, he was stopped and searched on Dame Street. Gardai found what a sergeant described as a "shank", but Ms McGhee said it was "broken tweezers".
Jennings accepted it was stupid to have it.
The accused was "a bit wild in his teens" but had no issue with heroin until he became homeless and "completely lost his way", Ms McGhee said.
He was now off drugs.
Judge Colin Daly adjourned the assault case. On other charges, he fined Jennings €300, gave him a two-week backdated sentence and put him on a one-year probation bond.