Sitting on a short flat terrace near the north-western base of a steep ridge, the structure commands a decent view north-westward and north-north-eastward across the Burren landscape, yet it gives almost nothing away. There is no visible entrance, the interior is level and largely featureless, and later field walls have been built directly onto sections of its perimeter, which is now heavily overgrown with brambles. A cashel, to use the term properly, is a stone-walled ringfort, the dry-stone equivalent of the earthen raths found elsewhere in Ireland, and this one survives as a subrectangular enclosure roughly 23 metres north to south and 21.5 metres east to west, defined by a broad externally faced bank of stone averaging about four metres wide and standing no more than a metre high in places.

The site was already recorded on the Ordnance Survey six-inch map of 1842, marked with hachures indicating an earthwork or enclosure, and it appeared again on the 1915 edition. What gives it real historical weight, however, is its company. Writing in 1897, the antiquarian W. C. Borlase counted twenty ringforts between Derreen West and Derreen East alone, and thirty-three on the broader slope that includes the north-western face of Knockauns Mountain. That is a concentration remarkable even by Burren standards, a region already well known for its density of early medieval settlement remains. T. J. Westropp, visiting shortly afterwards and publishing his observations in 1901, noted that several of these monuments had been largely levelled and rebuilt as sheepfolds, though he was nonetheless confident they were of genuine ancient origin. The cashel at Derreen fits that pattern: a structure old enough to be mapped in the nineteenth century, altered by subsequent agricultural use, and now difficult to read as anything other than a rough stone bank sinking slowly back into the hillside.