The constitutionality of existing legislation on the protection of national monuments will be the subject of a High Court case that may force the government to rewrite legislation and ultimately re-route the M3 motorway planned to run through the Tara Valley.

The legality of the planned M3 route will be debated in a hearing which begins on Thursday and is expected to last up to six days. Conservationists, led by campaigner Vincent Salafia, claim that the route runs directly through a national monument which should be given constitutional protection.

They will also claim that the National Monuments (Amendment) Act 2004 is unconstitutional as it weakened the role of the Oireachtas in debating the protection of national monuments, and centralised all decision-making in relation to the protection of such monuments to the environment minister.

Regardless of the outcome of the hearing, it seems likely that the construction of the motorway in Co Meath may face delays of up to two years due to conservationists' determination to take the case to the Supreme Court if necessary. This would be a major blow to the government, which had initially planned that the road would open later this year.

In affidavits supplied to the court by leading Irish archaeologists, it is claimed that the route of the motorway runs directly through the series of national monuments making up the Tara Valley. However, the government, Meath County Council and the National Roads Authority (NRA) argue that the national monument is located at the Hill of Tara and that the surrounding areas are not part of the monument.

The absence of legislation governing archaeological landscapes has been criticised by groups such as the Heritage Council, which argues that legislation in other countries protects the landscape surrounding a national monument and not merely the monument itself.

The High Court will hear claims that the motorway will cut the landscape of Tara in two by going through the valley which separates the hills at Tara and Skyrne. The NRA argues the national monument ends at the Hill of Tara, which will not be affected by the road.

Aside from the protection of landscapes, conservationists will argue that the amended National Monuments Act is unconstitutional because it centralised decision-making into the hands of the Minister for the Environment, weakening the role of the Oireachtas.

The Minister for the Environment, Dick Roche, currently has sole discretion to define the importance of national monuments, although he is obliged to take advice from the director of the National Museum, Dr Pat Wallace.

Earlier this year, Wallace stated his belief that the Tara Valley "constitutes an archaeological and cultural landscape which deserves the fullest and most generous archaeological protection".

He said that the region was the most important of its type in Ireland, "if not Europe".

That view was challenged by the government's chief archaeologist, Brian Duffy, who contended that it was not possible to link the surrounding areas with the Hill of Tara because the various archaeological sites were not all from the same time period.

NUI Galway archaeologists Conor Newman and Joe Fenwick, who both worked on the government-funded 'Discovery' programme which investigated the area, will testify in the High Court that the landscape must be viewed as one archaeological area. Newman and Fenwick will state that the planned route should never even have been considered for the motorway, given the immense importance of the area.