A cashel is simply a ringfort defined by a stone wall rather than an earthen bank, and this one encloses an interior roughly 38 metres north to south and 30 metres east to west. The outer facing-stones on the east side still stand to almost two metres, which gives a reasonable sense of the original mass of the structure, though the inner facing-stones are gone entirely and the wall's full width of about 1.25 metres can only be gauged at the entrance. That entrance sits midway along the south wall and is framed by two upright orthostats, the taller of them rising to just over 1.5 metres. Inside the enclosure, a small hut site occupies the north-east corner, and a spread of stones in the south-west may indicate where a second once stood. A further concentration of stones just inside the wall, east of the entrance, likely marks something structural as well, though its precise function is unclear. About 15 metres to the west, a circular cashel sits on the same slope, the two enclosures lying close enough to suggest a deliberate arrangement.

The broader landscape around Derreen adds considerable weight to this site. Writing in 1897, the antiquarian W. C. Borlase counted twenty ringforts between Derreen West and East alone, and thirty-three across the wider slope, including the north-west face of Knockauns Mountain. That density is striking even by Burren standards, a region already well known for its concentration of early medieval settlement remains. By 1901, T. J. Westropp had noted that several of the sites had been levelled and their stones reused as sheep folds, a fate common to many such structures across the west of Ireland, yet he was confident they retained their ancient origins beneath the later alterations. The cashel at Derreen appears on Ordnance Survey maps from 1842 and again in 1915, indicating it was a recognisable feature in the landscape well into the modern period. Set on a north-facing slope in rough pasture, the enclosure commands good views to the north-west and north-east, while higher ground to the east and south-west looks down over it, a topographical arrangement that would have been very much part of how its original inhabitants understood their place in this dense, stone-walled world.