The Minister for the Environment, John Gormley, has signed a temporary preservation order on a national monument known as Rath Lugh in the Tara/Skryne valley.
People opposing the route of the motorway through the valley say work on the project has damaged the monument, a claim that has been disputed by the National Roads Authority and the Department of the Environment.
The national monument at Rath Lugh lies 2.5km from the Hill of Tara. The fort is also close to the other national monument at Lismullen.
Mr Gormley ordered an assessment following concerns about damage to the monument and the placement of the motorway 50m closer to the site.
The assessment concluded it was not under threat from the road works but was under threat from the dense undergrowth and forest on top of it. Some of this scrub had been cleared as part of the M3 project.
Both Coillte, which owns the land, and the NRA, have been informed that the temporary preservation order has been issued to prevent any further work taking place at or near the site.
The move has been welcomed by those opposing the route of the motorway, who have said they had warned that Rath Lugh would be damaged by the M3.