An Bord Pleanála has ruled the M3 motorway can proceed along its original route and that the National Roads Authority can start work on the excavation on the national monument at Lismullen, Co Meath, before constructing the road on top of it.
Campaigners against the motorway route immediately said they would appeal the decision to the European Commission, which has already said it is investigating whether aspects of the M3 route may be illegal.
Campaigner Vincent Salafia
The semicircular enclosure at Lismullen, about four miles south of Navan on the existing N3, lies across the northbound lane of the proposed new M3 motorway. It is 80 metres in diameter and dates from between 380BC and 520BC. It is thought to have been some kind of ceremonial site.
The discovery was granted National Monument status in May, leading conservationists to claim it supported their contention that the Hill of Tara archaeological complex also included the Gabhra Valley area.
However, in one of his final acts of office, former environment minister Dick Roche used the National Monuments Act 2004 and signed an order of preservation by record, meaning the prehistoric henge would be photographed, sketched and measured before being razed to make way for the motorway.
In coming to its decision, An Bord Pleanála said it had considered the alignment and design of the approved road development and the modifications subject to which the road development was approved.
It also considered the directions issued by the Minister for the Environment to fully excavate and record the National Monument site.
Business lobby group Ibec welcomed the decision today.
Paul Sweetman of the Ibec transport executive 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. A decision has now been made and the integrity of an Bord Pleanala and its judgment must be respected. Focus should now be on implementation."
The TaraWatch campaign group, which is demanding that the site be preserved, has called on the Government to halt excavation of the site. There have been a number of protests at the Lismullen site in recent weeks, resulting in the arrest of some of the campaigners for public order offences.
The National Roads Authority (NRA) today said it respected the board's decision, while protesters said the news was disappointing.
Paul Sweetman, Ibec
"We're obviously disappointed but not surprised," campaigner Vincent Salafia of TaraWatch said.
"We're going to run to our lawyers now with a copy of the decision from An Bord Pleanála. TaraWatch will make an immediate appeal to the European Commission over the decision of An Bord Pleanala to allow the M3 motorway to be built over the newly discovered Lismullen national monument.
"The decision of the board was made without any public consultation or opportunity for independent assessment to be given," he said.
The discovery at Lismullen came just a day after the then transport minister Martin Cullen turned the sod on the €850 million road project.
The semicircular enclosure, which lies across the northbound lane of the proposed motorway and is around 2km from the Tara hill, is 80 metres in diameter and dates from between 380BC and 520BC.
The discovery was granted National Monument status and all works were halted at the site.
The European Union's petitions committee called on the Irish Government to halt work on the motorway and said a route review should be carried out.
Minister for the Environment John Gormley says he does have the authority to revoke his predecessor's decision without a material change in circumstance.
Earlier this week a report by a US academic called for the preservation in situ of the site because of its unique size and character.
Claiming the ruins were under threat from adverse weather, state archaeologists began excavations on the site earlier this month.
TaraWatch is now planning a Love Tara march in Dublin on September 15th, the second such gathering, to protest at the development.
Additional reporting: PA