Environmentalist Vincent Salafia is facing a legal bill of more than €600,000.
It follows a High Court ruling that he was liable for the costs arising from his unsuccessful challenge to the proposed routing of the M3 motorway near the Hill of Tara.
Mr Salafia has indicated, however, that he will appeal to the Supreme Court.
He had urged the High Court not to make cost orders against him, arguing that he had taken the case in the public interest and out of concern for the Hill of Tara.
