The cashel at Derreen, a ringfort built from dry stone rather than an earthen bank, is the middle of this trio. Roughly circular and nearly 39 metres across, it is largely overgrown, its stony bank reduced to between 0.4 and 0.8 metres above the present ground surface on the outside, with inner facing-stones surviving in places to just 0.2 metres. The original entrance has been lost entirely, a more recent field wall having been built along the outer edge of the ancient structure in most sectors. A small circular walled outline survives near the north-north-west edge of the interior, though when it was last inspected in 1997 the vegetation was too dense to allow a closer look.

The density of these monuments in this corner of the Burren is genuinely unusual. 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-west-facing side of Knockauns Mountain. A few years later, T. J. Westropp noted in 1901 that several of them had been substantially levelled and then rebuilt for use as sheep folds, though he was confident the underlying structures were of ancient origin. The field system connecting the three cashels at Derreen may itself be partly older than the cashels, suggesting that farming and settlement here had a long, layered history before any of these walls reached their present form. The 1842 six-inch Ordnance Survey map records the cashel, hachured in the cartographic shorthand of the period for earthworks, and it reappears on the 1915 edition marked with a solid line.