Police are hunting vandals who paint-bombed the 5,500-year-old standing stone on the Hill of Tara.
In the second attack in as many years on the ancient Lia Fail - also known as the Stone of Destiny - red and black paint was spattered over the monument overnight.
Garda confirmed they are treating the incident as criminal damage and have appealed for witnesses to come forward.
Dr Jonathan Foyle, of the World Monuments Fund, has referred to Tara as "the equivalent of Stonehenge and Westminster Abbey all rolled into one".
The desecration of the sacred granite stone has sparked outrage, with Heritage Minister Jimmy Deenihan denouncing the vandals as shameful.
"I condemn in the strongest terms the damage that has been caused to one of our most iconic ancient monuments," he said.
"This act of mindless vandalism, on one of our premier archaeological sites, is truly shameful."
Mr Deenihan said the ancient monuments on the Hill of Tara are cherished not just in Ireland but internationally.
"I call for anyone with any information about who may have been responsible to inform the Gardai," he said.
The monument - celebrated as one of the wonders of Tara - is believed to have been a coronation stone where the ancient High Kings of Ireland were crowned.
It features extensively in ancient texts.
Detectives are to carry out a forensic examination of the site.
The damage was first noticed by official guides from the Office of Public Works this morning.
In 2012 damage was caused in 11 places on all four faces of the standing stone after vandals took a hammer to it in an attack which provoked debate about increased surveillance.