Protesters blocked access to the national monument at Lismullin, Co Meath, yesterday just before controversial excavation work began at the site.
Fifteen demonstrators formed a human chain which blocked the only access road into the site. They prevented workers going on site for 45 minutes yesterday morning, but were moved on after being threatened with arrest by gardaí.
Excavation work on the national monument at Lismullin was given the go-ahead last Thursday by Minister for the Environment John Gormley's Lismullin advisory committee.
Mr Gormley's chief adviser on Tara, Dr Conor Newman, said the site was "literally washing away" from the deluge of rain this summer.
He said it was the unanimous decision of the committee that the excavation should take place as the Lismullin henge was showing signs of rapid disintegration since it was uncovered earlier this year.
He told RTÉ's Morning Irelandprogramme that the disintegration of the henge was being caused by a combination of the unprecedented amount of rain and the gradient of the site.
"I would be much happier to say to you that the monument would be preserved in situ and that was my earnest desire myself until I inspected the site itself and saw the types of conditions under which it was trying to survive."
Dr Newman said that if Mr Gormley put a preservation order on the site, it would be of no use in the circumstances.
"We're at a situation now where, under ideal circumstances, the site would be covered over again and there would be no prospect of any development at all. The problem is that the site is not sufficiently robust to be covered at all, regardless of what might happen in the short term," he said.
The Lismullin henge was discovered earlier this year and the announcement of its discovery came just a day after the then minister for transport, Martin Cullen, turned the sod on the €850 million road project in May.
The semicircular enclosure, which has a diameter of about 80m, dates from between 380 BC and 520 BC and is thought to have been some kind of ceremonial site.
Initial excavations of the site also revealed a ring ditch, which is a prehistoric burial site, a souterrain, which carried water, and corn-drying kilns dotted around it.
Former minister for the environment Dick Roche used the National Monuments Act 2004 to announce the "preservation by record" of the Lismullin henge on his last day in office. It will mean that all valuable artefacts from the site will be preserved, but the site itself will be covered over to become part of the M3.
The 30-strong team of archaeologists began work at about 9am yesterday.
The initial work will involve the cleaning of individual features on the site.
That will be followed by a scale drawing and then the process of preservation by record will begin. The process is likely to take three months.
Protesters said their decision to move on when confronted by gardaí was taken because their numbers would have been diminished had any of them been arrested and bailed. "We had seven people arrested last month and a condition of their bail was that they couldn't turn up on site any more," said protester Aogán Delaney. "If they arrested the rest of us, it could wipe us out."
Meanwhile, TaraWatch has sent solicitors' letters to Mr Gormley, the National Roads Authority, Meath County Council and the Attorney General demanding that the excavation work be stopped.
TaraWatch says the National Monuments Act 2004 is contrary to EU law because it does not require a new environmental impact assessment following the discovery of the Lismullin site.
