Archaeologists believe the new monument discovered along the planned route of the M3 motorway may have been a royal site, probably used for open-air rituals at the same time that similar ceremonies would have taken place on the Hill of Tara just 2km (1.2 miles) away.
It is a late prehistoric circular enclosure, but unlike similar enclosures at Emain Macha, seat of the kings of Ulster, in Co Armagh, and Tara, which had many phases of building and rebuilding, this new site seems to represent just a single period of use for such rituals.
Why this is the case is one of the many questions the discovery has raised. Radiocarbon dating will reveal later this month how old it is; it is believed to be from the late Bronze Age/Iron Age.
The outer circle is about 80m in diameter and it lies across what would be the northbound lane of the €850 million motorway.
The future of the site, and particularly whether archaeologists will be told to excavate it, is in the hands of the Minister for the Environment, Dick Roche.
During a guided tour of the site for the media yesterday, about a dozen members of the Campaign to Save Tara group arrived and hung large banners calling for the road to be re-routed.
"We believe the Government is not prepared to accept the significance of what has been found here," said its spokesman Michael Canney. The protesters took pictures and video footage of the site and one woman claimed there was a "cover-up" of the find.
The Department of the Environment denied claims that the Minister has made a decision and said yesterday the process was ongoing. It is understood Mr Roche is awaiting a response on the find from the director of the National Museum, Pat Wallace, which is expected to come today.
Senior archaeologist with the National Roads Authority Mary Deevy said there were a number of factors which hint at the likelihood that the site was of great significance and potentially a national monument.
The first discovery was an arc of stakeholes which had once held wooden poles. This led to further work and the discovery of a large outer circle of stakeholes.
Archaeologists then found signs of an east-facing entrance and a smaller enclosure within the larger one. It is an exact circle with closely spaced stakeholes. The site is at the bottom of a natural basin and does not appear to have been roofed.
"It didn't appear to be defensive, was perfectly circular - we checked that on computer - and there is this circular enclosure perfectly centred in the middle of the larger one. When we put the plan of the enclosure together and looked at it, it was like 'Eureka!'," said Ms Deevy.
"It reminds us of Emain Macha and Dún Ailinne, which are major royal sites. They are sister sites of Tara where there was millennia of activity. This site is different, it is very simple but appears to have been built just once and we can't say how long it was here for," she added.
The discovery marks the conclusion of nearly two years of archaeological work on the planned route of the motorway.