The National Roads Authority is insisting that construction work on the controversial M3 motorway in Co Meath is not damaging a protected national monument at Rath Lugh.
Campaigners against the road claim the work is undermining the ancient fort around 2.5kms from the Hill of Tara.
A protestor has chained herself inside a tunnel dug into the hill in an effort to prevent the work from going ahead.
The Gardaí are currently monitoring the situation.
The NRA, meanwhile, says the construction work is not encroaching on the footprint of Rath Lugh and is necessary in order to install a wall that will protect the monument.