The Supreme Court is expected to give its third judgment today on the controversial routing of the M50 through the site of Carrickmines Castle.
Although the Dublin motorway extension has gone ahead, campaigners claim an outstanding constitutional ruling on the saga could have a major impact on the disputed M3 development.
A separate appeal on those plans, backed by protesters who say it will destroy much of the archaeological heritage around the ancient Hill of Tara, is to be heard in the Supreme Court in October.
The current Carrickmines case was taken by Dominic Dunne, of the "Carrickminders" group, which occupied the castle site in 2002.
The judgment is expected to rule on whether authorities have a duty to protect national heritage.
"This will be an historic day of reckoning," declared Vincent Salafia, who is driving the M3 challenge.
"The Government changed their own law to facilitate the motorway at Carrickmines, even after two Supreme Court decisions said they were acting illegally.
"The new law is also being used to run the motorway through (the Hill of) Tara, and while Carrickmines Castle may be ruined forever, it will not have been in vain if Tara can be saved," he said.