Gardaí are investigating vandalism to the Lia Fáil - the standing stone at the top of the Hill of Tara in Co Meath.
A garda spokesman confirmed that green and red paint was poured over the stone overnight.
At least 50% of the 5,000-year-old granite has been covered by the paint.
In 2012 the stone was damaged when pieces of it were hacked off with an ax.
Minister for Arts, Heritage and the Gaeltacht Jimmy Deenihan described last night's vandalism as "truly shameful".
"This is an act of cultural vandalism," said Ian Doyle, archaeologist and head of conservation with the Heritage Council.
He said local people and others "have a close relationship with the Hill of Tara and this is an assault on their sense of pride and the respect, love and affection they have for it."
Gardaí have appealed for anyone with information to contact the Garda Confidential Line on 1800 666111.