A cashel is a ringfort built primarily from stone rather than earthen banks, a form common in the west of Ireland and typically associated with early medieval settlement and farming. This one is subcircular, measuring roughly 23.7 metres east to west and 21 metres north to south internally, enclosed by a broad stone bank that survives to a height of around a metre at its northern side. Facing-stones, the dressed inner and outer surfaces that would once have given the wall its vertical faces, occasionally break through the grass, but nowhere rise more than about 20 centimetres. To the south the bank has been levelled entirely, and a modern field wall runs across and along it from south to northwest, erasing any trace of an original entrance.

What makes the site's setting genuinely peculiar is the company it keeps. Writing in 1897, the antiquarian William Copeland Borlase counted twenty cashels between Derreen West and Derreen East alone, and thirty-three along the broader slope that includes the northwest-facing side of Knockauns Mountain. That is a concentration unusual even by Burren standards, a region already noted for its density of early medieval remains. By the time Thomas Johnson Westropp surveyed the area in 1901, several of these structures had been substantially levelled and rebuilt as sheepfolds, their stone conveniently to hand for a practical purpose. Westropp was nonetheless confident they were of genuine ancient origin, repurposed rather than invented, their original form still legible beneath the agricultural overlay. The Derreen cashel fits precisely that pattern: old enough to appear on the Ordnance Survey six-inch maps of 1842 and 1915, where it is marked with the hachured symbol denoting an earthwork or enclosure, yet worn down far enough that its early medieval character is now more a matter of record than of visual impact.