Eddie Delaney (27) had had a dispute with the man and reacted "inappropriately" by taking the golf club on the bus, it was claimed.
Judge Treasa Kelly adjourned the case for a probation report.
Mr Delaney, of Newtown Drive, Ayrfield, Dublin 13, pleaded guilty to producing a weapon capable of inflicting serious injury.
Dublin District Court heard the incident happened on the number 15 bus route on November 27, 2019.
A man went to a garda station that evening and said he had been on a bus on the Malahide Road.
The court heard the accused had got on the bus at the Darndale roundabout and proceeded to assault a man with a golf club.
Gardaí viewed CCTV footage which showed Mr Delaney striking the man with a golf club and elbowing him a number of times into the head.
Other passengers were present on the bus at the time, a garda said.
Defence solicitor Ruth Walsh said Mr Delaney had not been charged with assault.
The court heard the injured party had not wanted to make an assault complaint against Mr Delaney.
It had been a "very out of character incident" for Mr Delaney, Ms Walsh said.
"He had a dispute with this gentleman and he reacted completely inappropriately," she said.
She said the accused was "entirely remorseful" and had been forthcoming in his dealings with gardaí, presenting himself to them voluntarily.
Mr Delaney worked in the office of a logistics company and had not come to the attention of the gardai since.
He accepted it must have been a "terrifying incident", Ms Walsh said.
Judge Kelly ordered a probation report and adjourned the case to a date in June.