The High Court has begun hearing an action by conservationists seeking to have the M3 motorway re-routed from the present route through the Tara/Skryne Valley.
The action is being defended by the State.
The Hill of Tara is a National Monument, but one of the key questions in this legal argument is whether that status is limited to the hill itself or extends through the valley below where the M3 motorway is routed.
In the High Court today, conservationists began their legal challenge to the decision of the Minister for the Environment, Dick Roche, to allow archaeological sites along the route to be excavated, effectively allowing the motorway through the valley.
Senior Counsel Gerard Hogan told the court that they will argue that Mr Roche used the wrong section of the National Monuments Act in making his decision, and that the act is unconstitutional on two grounds.
These two grounds are that the act removed all power from the Oireachtas and that the State has not provided adequate protection for national monuments.
The State is expected to argue that Mr Roche did not err in making his decisions and that the act is constitutional.
The case before Mr Justice Smyth is expected to last several days.
Campaigners opposed to the present route of the M3 motorway want Mr Roche to reverse his previous decision and move it away from the Tara/Skryne Valley.
They claim the motorway would destroy a unique archaeological landscape.
The minister has refused to re-route the M3, pointing out that the proposed route was approved by Meath County Council and An Bord Pleanála and is backed by many in the locality.