Dún Laoghaire Rathdown County Council has been granted an injunction by the High Court preventing protesters from occupying lands at Carrickmines that have been acquired for the south-eastern motorway.
Protesters have claimed the ruins of Carrickmines Castle are located on the motorway route and should not be interfered with.
But Mr Justice Paul Gilligan ruled in favour of the Council today and ordered that the injunction be posted on all entrances to the site and every 100 yards on the perimeter wall.
It is expected that the castle's perimeter wall will be removed in the coming days to make way for the new road.
The council took proceedings against two of the protesters, Mr Vincent Salafia and Mr Gordon Lucas, and other unnamed protesters, aimed at clearing the site for the construction of the road.
The council demanded that the protesters, who style themselves as "Carrickminders", stop "trespassing and interfering with the work of the county council's agents".
Last week, up to 30 protesters blocked archaeologists from removing part of a 12th-century revetted fosse (stone-lined ditch), that is to be preserved nearby. Once this is completed, work on the motorway will begin on the site.
Reacting to today's ruling the "Carrickminders" vowed to continue their fight to save the castle ruins.
"Carrickminders see today's result as a victory. We have succeeded in bringing the situation into the national and international spotlight. In addition, we have ensured that an unfinished excavation was brought closer to completion using Non-Violent Civil Disobedience. The dig is still unfinished," a statement said.
"Gordon and Vincent will continue the fight to save Carrickmines Castle, which is just beginning."