Most ringforts, which are circular enclosures of earthen banks built primarily during the early medieval period as defended farmsteads, have long since lost any trace of their original entrances. Here, a passage nearly eight metres long, lined with roughly coursed, punch-dressed stonework and covered by six of its original lintels, cuts through the inner bank and opens into a circular interior where old cultivation ridges still run in faint north-to-south lines across the grass. Two concentric banks, separated by a shallow fosse, a ditch dug to reinforce the defences, enclose a central area forty-two metres across, and a large outer fosse seven metres wide and two and a half metres deep rings the whole structure. Beneath the south-eastern quadrant, a souterrain, an underground stone-lined passage used for storage or refuge, adds another layer of complexity to the site.

The entrance had long since collapsed and been blocked up by the time the Office of Public Works undertook excavation and reconstruction between 1983 and 1984, work published by C. Manning in the Journal of Irish Archaeology in 1987 and 1988. During excavation, six of the original eight lintels were recovered along the passageway, two still sitting in position. Behind the main passage walls, excavators found secondary masonry faces that had served as retaining walls, and a series of post-holes and shallow gullies along the passage floor suggested at least two, possibly three, sets of doorways had once controlled movement through the entrance. An external stone façade over four metres long flanked the outer face of the passage. Following the dig, the façade and passage were rebuilt using as much of the original material as possible. Manning concluded that the masonry dated to within a century or two of the year 1000 AD, and that the quality of the construction and the unusual size of the enclosure suggested Cahirvagliair was built for someone of considerable standing, possibly even as a royal residence. A smaller ringfort lies roughly 350 metres to the east-north-east, though it was partially removed by road works.

Cahirvagliair is a National Monument in State care, and the reconstructed entrance passage remains the most immediate thing to seek out on a visit. The passage itself gives a rare, physical sense of how controlled and deliberate entry into such an enclosure would have been, with lintels overhead and dressed stone walls to either side, the interior opening out beyond a sill stone that has sat in the same place for roughly a thousand years.