Stephen White (31) had denied the assault, claiming the officer's male colleague had smashed his head off the door of the patrol car and "flung" him into the back seat.
He apologised if he accidentally "tipped" Garda Jennifer Brogan, who was four months pregnant at the time, but said he didn't think he had.
The court heard White made drunken threats to the female garda and her male colleague, telling them "you'll f***in regret this" and "I'll have you".
Finding him guilty, Judge Anthony Halpin described the assault as "disgraceful" and ordered White to complete 220 hours' community service in lieu of three months in prison.
White of Tymonville Road in Tallaght, was found guilty before Tallaght District Court of threatening and abusive behaviour, obstructing Detective Garda Michael McGrath and assaulting Garda Jennifer Brogan.
The incident took place on the Tallaght bypass on September 10 last.
Garda McGrath said he stopped White crossing the Tallaght bypass around 8.20pm for the purposes of a drugs search, as White had turned away when he saw gardai and appeared to be fidgeting in his pocket.
Garda McGrath claimed White was immediately aggressive, shouting "f*** off, my da's a garda, take me back to the station".
The court heard White further shouted: "f*** off, you'll f***in regret this, and "you'll feel my wrath".
Garda McGrath said White struggled with him and refused to allow him to put on the handcuffs.Garda Brogan said she was placing White in the back of the patrol car when he sat back and kicked out, hitting her in the stomach.
Garda Brogan, now eight months pregnant, said she was four months pregnant at the time, and was worried at the time about her unborn child.
In his evidence, White claimed he asked gardai to bring him to the station for a drugs search as it was embarrassing being searched at the side of the road.
He also claimed Garda McGrath "jumped on him" and smashed his head off the door of the patrol car. When they arrived at the station, White claimed he was grabbed by the legs, pulled out of the car and dropped on his back on the ground.
Defence solicitor Kevin Tunney said White is an unemployed father of two. Mr Tunney said he had apologised to Garda Brogan, saying any assault was accidental.