"This would be unusual for a prehistoric settlement. They are usually found on hilltops."
National University's Fenwick agreed that the enclosure seems to have had a ceremonial link to Ireland's prehistoric royal sites.
"The thing that gives it away is this funnel-shaped approach from the east between the outer and inner enclosures," Fenwick said.
"This is a recurring feature at all the major royal sites where you have a circular temple."
Ceremonies and worship performed at these monuments may have been related to cosmology and royal inauguration, he said.
(Watch a related video: "Ireland's Mysterious Newgrange Tomb.)
The site has yet to be precisely dated, but most experts believe it was in use some time after 1000 B.C.
"No matter what date the monument turns out as, we'll be able to link it to some phase of activity on the Hill of Tara," NRA's Deevy said.
Will the Road Go On?
"The chosen motorway route, unfortunately, goes right through the heart of this landscape," Fenwick added.
NRA officials have declared the Lismullin site a national monument, but this does not ensure its protection.
Legislation introduced in 2004 allows for national monuments to be "destroyed" if the environment minister considers such action "in the public interest."
"A lot of people think that this is what's going to happen," Fenwick said.
The Irish government is currently considering its options. Reports suggest the National Museum of Ireland in Dublin is urging a complete archaeological investigation of the site.
Meanwhile, activists are questioning why the Lismullin monument wasn't detected earlier, before the route was given the go-ahead, and are calling for the road to be rerouted.
Vincent Salafia of the nonprofit historic preservation group TaraWatch said the ancient enclosure is "without doubt a national monument of world significance. It would be a sin to demolish it."
Fenwick, the National University archaeologist, said: "It's not enough to reroute just around this particular site. People would argue that the motorway should be rerouted out of [what was] the royal domain of Tara altogether."
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