Irish actor Stuart Townsend has joined the campaign to save the Hill of Tara in Co Meath.
The “Save the Tara Skyrne Valley” group claims that the historic site would be damaged by the construction of the M3 motorway.
Mr Townsend has invited the public and several Irish celebrities - including U2, Sinead O'Connor, Neil Jordan and Jim Sheridan - to join him at the site today to highlight opposition to the road.
The gathering, called Feis Tara Skryne, is to get underway at Skryne Abbey at lunchtime where historians will be on hand to give tours.
The demonstration is to move on to the Hill of Tara at 3pm where poets, religious leaders and musicians will address the crowd.