A cashel is a stone-walled ringfort, the dry-stone equivalent of the earthen raths found across much of Ireland, and this one measures about twenty metres across internally. Its enclosing bank is only a fraction of a metre high on the interior, but the external facing-stones on the north-east to southern arc still stand between eighty centimetres and a metre and a quarter, suggesting the structure was once considerably more imposing. A small rectangular annexe projects from the north-east, and a curving wall trails away to the south-east; field walls running south and north-west may belong to the same original arrangement, though the whole complex has the quietly eroded look of something that has been quietly losing ground for a very long time.

What makes the Derreen cashel worth singling out is less the monument itself than its company. Writing in 1897, the antiquary W. C. Borlase counted twenty ringforts between Derreen West and East alone, and thirty-three on the wider slope that includes the north-west face of Knockauns Mountain. That density is striking even by Burren standards, a landscape already well known for its concentration of early medieval remains. A few years later, in 1901, T. J. Westropp noted that several of the forts had been largely levelled and rebuilt as sheep folds, though he was satisfied they retained their ancient origins beneath the agricultural reworking. The cashel at Derreen sits within that broader picture: one node in what was evidently a densely settled early medieval landscape, its closest neighbour a rectangular cashel with a small hut site in its north-east corner lying only about fifteen metres to the east.