Conservationists have threatened legal action if the Minister for the Environment, Mr Roche, gives the go-ahead to excavation of the proposed M3 motorway through Co Meath.
The Save Tara Skryne Valley group made the warning after they presented the Minister with a petition listing 10,000 signatures asking him not to proceed with the motorway plan.
The group's public relations officer, Mr Vincent Salafia, said its legal advisers were preparing a constitutional challenge to the National Monuments Act.
Nobody wanted "a repeat of Carrickmines", Mr Salafia said. But he added: "If the Minister gives the go-ahead during the next couple of weeks, we'll be into court immediately."
Although Mr Roche did not accept the petition in person, the group still hopes for a meeting with him before a decision is made.
In a letter accompanying the petition, the Minister was urged not to approve excavation, because the proposed route "unnecessarily traverses and partially demolishes the national monument of the Hill of Tara".
The letter claims that a decision to proceed would be "contrary to Irish and European law because it [ would allow] Ireland's premier national monument to be excavated and dissected by a motorway project when there were alternative routes considered that did not encroach on the monument.".
The letter and petition were accompanied by letters of protest from British archaeologists, the Archaeological Institute of America and Irish and international academics.
"All the expert advice worldwide is: just don't go there," Mr Salafia said.
He acknowledged, however, the people of Meath were "very much divided" on the motorway plan, because traffic problems were a source of frustration.
