Stephen Kennedy (28) left Jim Kelly with cuts and bruises to the face during the assault.
The defendant and his older brother Wayne (31) pulled up in front of another man's car as he was driving Mr Kelly home, and then both of them attacked Mr Kelly while he was sitting in the passenger seat.
Judge John O'Connor ordered Kennedy to complete 200 hours community service in lieu of six months in prison.
His brother Wayne Kennedy was sentenced to six months in prison last week.
The defendant, of Beechurst in Bray, admitted before Dun Laoghaire District Court to seriously assaulting James Kelly.
Garda Sergeant Peadar McCann said that Mr Kelly left the Ramblers Rest pub with the manager Pat Rice on September 23, 2011.
They were driving on Church Road in Ballybrack when a car driven by the defendant and his brother pulled up in front of them, blocking the road.
Sgt McCann said Mr Rice locked the car doors, but he then got out of the vehicle in an effort to diffuse the situation.
The sergeant said Kennedy got into the driver's seat and punched Mr Kelly in the eye and pulled him out of the car.
Sgt McCann said Kennedy then stood back, and his brother took over, fracturing Mr Kelly's finger.
Defence lawyer Brian Gageby said Kennedy has never been in trouble before, and there was a history between the parties as the victim is the defendant's sister's ex-partner.
Mr Gageby said Kennedy "simply lost it" on the day but that he subsequently co-operated fully with gardai and admitted to his part in the assault.
The judge ordered Kennedy to complete 200 hours community service in lieu of six months in prison.