The site at Castlenancy is a rath, the most common type of early medieval settlement enclosure in Ireland, typically consisting of a raised circular bank with a ditch, or fosse, running around the outside. This one measures roughly 45 metres in diameter and survives in fair condition, its bank and external fosse still legible in the landscape despite centuries of agricultural activity around it.

On the western side, a causewayed entrance gap some 2.5 metres wide remains visible, wide enough for people and animals to pass across the fosse and into the enclosed interior. That kind of detail, the specific width of an entrance preserved across more than a thousand years, has a quiet strangeness to it. More unusual still is what lies beneath the surface of the north-eastern quadrant: a souterrain, which is an underground stone-lined passage or chamber, typically associated with storage or refuge, built in the early medieval period and often found in association with ringforts of this kind. Souterrains are not rare in Ireland, but their presence always adds a subterranean dimension to what otherwise looks, from a distance, like a simple grassy bank in a field.