A republican anti-water charge protester sat on a road in front of a patrol car last July, preventing it from transporting a prisoner to a garda station.
Donall O Ceallaigh (32) was ordered to move and walked away briefly, but returned and stood in front of a garda car.
O Ceallaigh, of Emmet Road, Inchicore, was protesting against the installation of water meters in Foxrock at the time.
Judge Anne Watkin ordered him to complete 60 hours community service in lieu of one month in prison.
O Ceallaigh, who was fined for throwing an egg at then Tanaiste Eamon Gilmore's car in 2012, indicated he was unwilling to do community service.
Judge Watkin said he can be assessed, and the default order was in place if he was unsuitable or failed to co-operate.
O Ceallaigh, also known as Daniel Kelly, was found guilty before Dun Laoghaire District Court of failing to follow the directions of gardai and obstructing gardai at South Park in Foxrock on July 14 last year.
Sergeant Tom Doran said gardai had arrested another individual and were taking him away in the patrol car. Sgt Doran said he directed a number of people to move out of the way of the patrol car.
He said O Ceallaigh stood in front of the car, and then sat down on the road, putting his shoulder under the bonnet.
Sgt Doran told O Ceallaigh to move, which he eventually did, but he continued to stand in front of the patrol car, walking very slowly in front of it.
Defence lawyer George Burns claimed O Ceallaigh had a right to a peaceful protest, and did comply with garda directions to move. However, Judge Watkin found O Ceallaigh guilty.
Mr Burns said father-of-four O Ceallaigh is on a CE scheme.
Commenting on his Facebook page afterwards, O Ceallaigh - a supporter of Republican Sinn Fein - said he had been "quietly confident" the case would be struck out, as he had been "protesting in a peaceful manner".
"What I didn't expect was the politically motivated hearing I was subjected to. The judge was totally biased and in favour of the State at every turn, dismissing every argument my legal team presented," he posted.