The World Monuments Fund has declared the Hill of Tara to be one of the 100 most endangered heritage sites.
Two other Irish sites, Vernon Mount in Cork and Richhill House in Armagh city, have also been declared 'endangered'.
The independent body said that human activity has become the greatest threat of all to the world's culture and includes Tara under a section entitled 'economic and development pressures'.
It added that Tara is at risk because 'cities and suburbs are spreading unchecked'.
'Man is indeed the enemy', said WMF president Bonnie Burnham.
She added: 'Just as we caused the damage in the first place, we have the power to repair it, by taking out responsibility as caretakers of the world's cultural heritage seriously.'
Campaigners seeking to re-route the M3 motorway around the Hill of Tara believe the WMF designation should lead to a change in Government policy.
TaraWatch is calling on the Green Party to do all in its power to have the road re-routed in its discussions with Fianna Fáil on the formation of the next Government (read more about those negotiations here).
The designation of the Hill of Tara was made following an application by the TaraWatch campaign earlier this year and an examination by an international panel of experts.
Vincent Salafia of TaraWatch is travelling to London today for talks with officials from the World Monuments Fund, which could lead to funding for an independent archaelogical report on the proposed motorway.
Since its beginnings, the Fund has given €35m in grants to 214 sites in 74 countries.
The group has already said it is committed to taking legal action if Environment Minister Dick Roche allows the National Roads Authority to destroy the recent major find at Lismullen.
A spokesman for the Minister said he would not be commenting on the announcement from the World Monument Fund until he had received and read material from the organisation.
The Fund first produced the list in 1995 and says that 75% of its Watch sites have been saved or are now out of danger thanks to timely intervention.
Also listed in the economic and development pressures section are Machu Picchu, Peru; St Petersberg Skyline, Russia; Old Damascus, Syria; Hasankeyf, Turkey; and Rock art at Dampier, Australia.