Nobody knows exactly when it was built. The walls survive only as low, grass-covered ridges, nowhere rising more than a quarter of a metre above the ground, yet their rounded corners and the slight thickening of the southern wall to around 1.2 metres still trace a legible plan across the ground.

A cashel is a stone-walled enclosure, roughly equivalent in function to an earthen ringfort, and was a common form of enclosed settlement in early medieval Ireland, though examples were constructed across a broad span of centuries. This particular cashel at Clooneen contains at least two structures of this kind. The western house, pressing itself against the interior of the enclosing wall, is matched by a second building of similarly uncertain date sitting roughly 18 metres to the south-east, in the opposite corner of the same enclosure. The pairing suggests the cashel once supported a small domestic or agricultural community within its walls, each building making use of the shelter and security that the perimeter provided.