Environmentalist Vincent Salafia is facing a €600,000 bill arising from his unsuccessful challenge to the proposed routing of the M3 motorway near the Hill of Tara.
Mr Salafia said he was facing the prospect of bankruptcy in the wake of Judge Thomas Smyth's ruling at the High Court.
"If I don't pay up fairly quickly, they will bring bankruptcy proceedings against me. It obviously will be a huge factor in my ability to own or direct a corporation. It will affect my ability to make a living, to provide for my family, so really it's basically a very punitive judgement."
However, Mr Salafia said he would be appealing to the Supreme Court and added that the costs might not be awarded against him if he was successful.
The environmental campaigner, from Dodder Vale, Churchtown in Dublin, claimed that he took the action on public interest grounds and that there was no personal gain involved.
"A national survey last year showed that 70% of Irish people were against the route of this road and wanted this road re-routed," he said.
But in the High Court, Judge Smyth said he did not accept that Mr Salafia had been acting in the public interest.
He said the Trinity College student had failed to establish that there were any national monuments on the route of the €600 million M3 motorway between Clonee and Kells and added that he had not taken part in the oral hearings held by Bord Pleanala into the routing of the motorway.
"If he was bona fide motivated by public interest concern, it is inconceivable to me that he failed to avail of these many opportunities," he said.