A souterrain is a man-made underground passage or chamber, typically stone-lined and roofed with large slabs, built during the early medieval period for storage, refuge, or both. Here, the above-ground archaeology has fared only marginally better.

The site sits on a slight south-east-facing slope and takes the form of a roughly circular ringfort measuring around 39 metres in diameter. A ringfort, to use the Irish term ráth, was a farmstead of the early medieval period enclosed by one or more earthen banks and ditches. At Knockereen, an inner bank survives along with traces of an intervening fosse, which is simply a ditch, and an outer bank running from the north to the east arc of the circuit. The monument is poorly preserved, and a pathway cuts across it at both the north-east and south-west, compounding the damage. Inside the interior, two circular stone-lined depressions, each roughly 1.4 metres in diameter, have been noted; these may represent collapsed animal pens, though nothing certain can be said. A related enclosure lies approximately 150 metres away.