A man nicknamed 'Big Al' beat up a 15-year-old boy and his 4ft 11in father after a falling out between their families.
Alan Brogan (43) "battered" the teenager's face and jumped on the boy's father, leaving him with a black eye.
Dublin District Court heard both victims were much smaller than the defendant.
Brogan, a grandfather and father-of-one of Annesley Close, Ballybough, had pleaded not guilty to assaulting the boy and his father on July 23, 2015.
Judge Timothy Lucey found him guilty and fined him €500.
The court heard the dispute dated from an incident that happened on the previous New Year's Eve.
The boy told the court he was outside his home on July 23 when Brogan arrived in a car driven by another man.
"When I get your Da, I'm going to kill him," Brogan said, according to the boy, who gave evidence via video link.
"What are you talking about?" the boy asked Brogan, who said: "You know what happened, you were there that night."
The accused made allegations, which the boy denied, then got out of the car, grabbed him by the face and punched him in the mouth.
He was hurt and felt in shock, and was left with a bloody lip.
Brogan then ran over to the boy's home and attacked his father. He jumped on the man, who stumbled back on top of a clothes horse.
The accused then began punching him, leaving him with a black eye and marks on his legs, arms and face,
The father told the court he was 4ft 11in tall while his son was 5ft 1in and he estimated the accused was over six feet tall.
Brogan claimed in evidence that, when he arrived, the boy told him "Yous are all getting done" and threw punches.
He said he pushed the teenager away and alleged that the boy's father and the father's partner "jumped on" him.
He said there was a disagreement and the three were "roaring at each other".
He denied he then tried to kick their door in when they returned inside. He admitted he had been drinking earlier.
Judge Lucey said he did not accept that the victims had "made it all up".
The accused had no previous convictions and worked in a security firm.