Its overall outline is roughly heart-shaped, stretching some 210 metres on its longest axis, and it is defined not by a single boundary but by a layered system of earthen banks and intervening fosses, the ditches that typically accompany such ramparts. Within that outer circuit sits a D-shaped inner enclosure, concentric with the main earthworks along its curved side. Seven radial banks then connect the inner enclosure to the outer defences, dividing the interior into distinct zones, much as the spokes of a wheel divide a rim. One of these radial lines on the western side widens into two parallel banks before stopping short of a small circular enclosure, saucer-shaped inside and scattered with loose stone. A raised square platform, measuring roughly 17 by 16 metres and standing about a metre above the surrounding ground, sits in the northeast. What makes the overall plan so legible is precisely how intact most of it remains, though the eastern half of the D-shaped inner bank, still visible on the six-inch Ordnance Survey map of 1842, has since been levelled.

The site is locally known as Cill-ma-cu, a name recorded by MacCarthaigh in 1975, and tradition holds that St Carthage, said to have been the son of a man named Cu, founded a small settlement here. The church site and graveyard of Kilmacoo occupy the western interior and were likely, as M. F. Hurley noted in 1983, an original internal division of the enclosure rather than a later addition. The main entrance to the whole complex lies to the south-southeast, flanked by stone pillars, and a pair of parallel banks leads from it directly to the graveyard entrance, 8.1 metres apart, creating a kind of formal approach corridor. To the north, a deliberate break in the inner bank, with a stile crossing the outer one, provides access towards a holy well about 110 metres away to the northwest, suggesting the well was integrated into the site's organisation from an early stage. A possible souterrain, an underground stone-lined passage sometimes used for storage or refuge in early medieval Ireland, has also been identified within the interior.