The stone was discovered within an early medieval souterrain, an underground structure that may have been used by local inhabitants to defend themselves against Viking raiders, the excavation team reported.
Dating to around the 10th century A.D., the souterrain was probably constructed using the broken megalith as building material.
"The souterrain builders robbed or quarried the stone from a Neolithic [Late Stone Age] monument," Deevy said.
"Souterrains are common in Ireland, and it's not unusual to have stones from earlier monuments reused on them," she added.
The rock art will eventually go on public display, according to Deevy, who describes the Lismullin site as "100 percent excavated."
The site was handed over to road builders on December 18, with construction work expected to start soon.
Campaigners who want the highway re-routed away from the Hill of Tara area have attacked the decision.
"Significant damage has already been done," said Vincent Salafia of Dublin-based protest group TaraWatch.
"But until the road is built on top of the site, I suppose there is still hope."
One of the "Top Discoveries"
Irish citizens opposed to the road project are currently fielding legal advice with a view to obtaining a court injunction to halt construction, the campaigner said.
The European Commission has reportedly criticized the Irish government for failing to properly reassess the impact of the road project after the ruins of the open-air temple were uncovered last year.
Under European law, the discovery should have triggered a so-called environmental impact assessment, Salafia said.
While the Lismullin site was declared a national monument, "this has made no difference whatsoever," he added.
NRA spokesman Sean O'Neill said the find led to a government-ordered review of the Lismullin site by a panel of independent experts.
The panel concluded that the excavations had been properly conducted "and that all appropriate actions had been taken," O'Neill said.
The concerns expressed by the European Commission related to Irish road projects in general and not specifically to the Lismullin site, he added.
Salafia said that as many as 40 archaeological sites have been uncovered along the route of the M3 highway.
"The bigger argument that's at stake is that Lismullin is connected to all these other 40 sites, and that they are all part and parcel of one single large national monument, which is the Hill of Tara complex," he said.
"If a new environment impact assessment were done, that's what would be shown, and that the motorway should go outside that complex rather than straight through it."
Lismullin's timber enclosure site was recently named one of the top ten discoveries of 2007 by the magazine Archaeology, published by the Archaeological Institute of America.
"Construction of the new M3 highway, meant to ease traffic congestion around Dublin, threatens not only the Hill of Tara's timeless quality, but also newly discovered archaeological sites in the surrounding valley," the magazine said.
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