The cashel at Derreen is one node in that cluster, an oval stone ringfort sitting on a flat terrace with rising ground to the south and open views across the landscape from west to east. A cashel is a type of ringfort built from dry stone rather than earthen banks, typically dating to the early medieval period and associated with farmsteads or small settlements. This one measures roughly 33 metres north to south and 30 metres east to west, its perimeter wall still standing to about a metre in external height, though heavily overgrown. Occasional facing-stones survive along the northern and eastern arc, and inside the enclosure, slightly west of centre, the remains of a circular hut foundation are still visible.

Two further cashels sit close by, one approximately 60 metres to the north-east and another around 180 metres beyond that. The field walls running between all three appear to belong to the same period of use, and some sections may actually predate the cashels themselves, suggesting a landscape that was being actively farmed and organised across generations. William Copeland Borlase, writing in 1897, counted twenty such enclosures between Derreen West and Derreen East, and thirty-three in total on the slope that includes the north-west face of Knockauns Mountain. Thomas Johnson Westropp, surveying the area a few years later in 1901, noted that several had been substantially levelled and their stones reused to build sheepfolds, though he remained confident of their ancient origins. The cashel at Derreen itself was recorded on the six-inch Ordnance Survey maps of both 1842 and 1915, marked with the hachuring that cartographers used to indicate earthworks and enclosures.

The density of monuments here is genuinely unusual. Most areas of Ireland yield the occasional ringfort scattered across farmland; this part of the Burren presents something closer to a fossilised agricultural landscape, where the enclosures, hut sites, and field boundaries still read as a coherent system rather than isolated survivals. Much of it lies beneath vegetation now, and the cashel at Derreen requires some patience to interpret on the ground, but the scale of what remains, once you begin to look across the slope, is quietly striking.