It is not dramatic from a distance, and it has no name beyond its townland, but it is a substantial thing: 110 metres across, defined by two earthen banks with a fosse, or ditch, running between them.

Enclosures of this type, formed by concentric banks and ditches cut into the earth rather than built from stone, are found at various points across Ireland, though their purposes and dates vary considerably. Some were defensive, some ritual, some used for enclosing livestock. At Carrownderry, the inner bank survives well along the northern through to the southern arc, while the outer bank and fosse are best preserved along the eastern and southern sections. Elsewhere, what remains is a degraded scarp, a softened slope in the ground where the original bank has slumped and spread over time. Two gaps in the circuit, one to the west and one to the east, may represent original entrances, though it is not possible to say with certainty which, or whether both, were always there.

The site sits just off the summit, angled so that it looks down over Williamstown to the southeast. That positioning, close to but not quite on the highest point, is characteristic of a number of Irish hilltop enclosures, and raises the same unresolved questions they all do: built by whom, used for what, and across what period of time. The earthworks are in fair condition, which in a landscape of working grassland is perhaps more than might be expected.