Archaeologists working on the new national monument discovered at Lismullen, near the Hill of Tara, believe it was probably an important ceremonial site.
They also believe it may have been made of 'wattle and daub', similar to the way baskets are weaved together.
The site consists of two circular enclosures, the largest 80m in diameter, the smaller one inside just 16m in diameter.
The larger outer wall could have been reasonably small, with the wall of the inside enclosure probably taller.
As the site is in a small hollow, archaeologists believe it would have allowed others to stand on the nearby ridges to watch the ceremonies taking place inside the enclosures.
Mary Deevy, Chief Archaeologist with the National Roads Authority, said the structure dated from somewhere between 1000BC to 400AD.
Samples have been taken and are being sent away for radio carbon dating. That will give a more precise period.
Archaeologists started working on the site at Lismullen last February, but the enclosures were only fully uncovered about a month ago.
The archaeologists then mapped the area and notified the Department of the Environment of the significance of the discovery.
Ms Deevy said there was a similar enclosure found by geophysical surveying on the Hill of Tara, which, at 200m in diameter, was much larger.