The man, who is in his 20s, had turned up at his ex-partner's house demanding to see their young son when the assault occurred.
He was also found guilty of assaulting his former partner after he pushed her up against the wall during the argument.
He apologised for his behaviour, saying it was completely wrong.
Judge Michael Coghlan ordered a community services report to see if the man was suitable to complete 120 hours community service after he brought €300 compensation to court for the girl.
The defendant previously admitted before Blanchardstown District Court to seriously assaulting the 14-year-old girl in the incident at a west Dublin address in December last year.
He was also found guilty of assaulting his former partner during the same incident.
Garda Sergeant Mary Doherty said the man turned up at his ex-partner's house around 8pm on December 16 last demanding to see his young son.
Sgt Doherty said there was a verbal argument with his ex-partner and during the row he pushed her against the wall.
The sergeant said the teenager tried to intervene in the dispute, to protect her mother.
Sgt Doherty said the defendant punched the girl in the face and pushed her into the stairs, leaving her with bruising.
The court heard the defendant has no previous convictions.
Defence solicitor Matthew Kenny said the defendant was very sorry for his behaviour.
Mr Kenny said the defendant and his ex-partner got involved in an argument when he went to her house to pick up his son. He said the teenager got involved because she was trying to protect her mother.
Mr Kenny said the defendant now has a good relationship with his ex-partner, and he is supporting his son and seeing him on a regular basis.
He said the defendant, who is employed as a retail worker, has a modest income but had saved €300 compensation for the teenager.
Judge Coghlan adjourned the matter to a date in December.