An Bord Pleanála has cleared the way for the M3 motorway to be built across the site of the national monument at Lismullin, Co Meath.
The move means potential delays to the construction timetable for the motorway can now be avoided, the National Roads Authority said yesterday.
However, opponents of the scheme said the board's decision ran contrary to EU law and warned they were considering a further challenge in the Irish courts.
They called on Minister for the Environment John Gormley to halt the destruction of the monument pending an EU determination on the legality of the National Monuments Act.
In its judgment, An Bord Pleanála said that as a result of the Government's direction that the monument be preserved "by record" rather than in situ, no material alteration to the road building scheme had arisen.
The board's ruling means that no new environmental impact statement is required following the discovery of the Lismullin artefacts and their designation as a national monument.
The ruling sets an important precedent should any of 38 sites of archaeological interest already identified in the original environmental impact statement subsequently be declared a national monument. It implies that as long as national monuments are not preserved on site, there may be no impact on road construction.
In giving its ruling, An Bord Pleanála said it had regard to an amendment to the National Monuments Act brought in by former minister Dick Roche. That amendment conferred on a minister the power to direct that a national monument should be "preserved by record" but not in situ.
Mr Roche exercised that power during his final days in office, and issued directions for preservation by record to the National Museum last June.
The board also said it had had regard to a statement from the National Roads Authority to the effect that there was no change necessitated to plans for the construction of the road, if the monument was preserved by record.
The board concluded that "as a consequence of the minister's directions no material alteration arises to the road development as approved".
Responding to the ruling yesterday, Vincent Salafia of the group TaraWatch said the campaign to preserve the Lismullin site was "far from over".
Mr Gormley was in Berlin yesterday but a spokesman reiterated the Minister's view that "all the decisions made in relation to the M3 had been made before the Minister took office".
The Minister had "absolutely no powers to interfere" in the statutory processes already in place.
Mr Salafia said a number of options were being considered by TaraWatch, including a judicial review of the board's decision. He said the amendment to the National Monuments Act introduced by Mr Roche was currently the subject of a complaint to the European Commission.
Mr Salafia said the EU Commission had already issued a "reasoned opinion" against Ireland on the matter. He believed it was wrong of An Bord Pleanála to rely on the relevant section of the National Monuments Act under these circumstances. "That might give us case to judicially review the board's decision" he said.
Mr Salafia said Mr Gormley had the power to delay the destruction of Lismullin until the EU issued a final determination.
He recalled that former minister for transport Martin Cullen had been able to halt the construction of the Waterford city bypass to ensure its route was altered, following the discovery of important Viking artefacts there.
The employers' body Ibec called for the An Bord Pleanála decision to be respected. Its transport executive Paul Sweetman said the development "of a modern Irish road network is vital and it is clear to all that the M3 is a much-needed piece of infrastructure".
