On the limestone uplands of the Burren in County Clare, ringforts, roughly circular enclosures built from earth and stone that served as farmsteads and defended homesteads during the early medieval period, are not unusual. Here, however, they accumulate in a way that arrested the attention of antiquarians over a century ago and has not quite been explained since.

In 1897, the scholar 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 flank of Knockauns Mountain. A few years later, in 1901, Thomas Johnson Westropp noted that several of those forts had been substantially levelled and pressed into service as sheep folds, their ancient stonework quietly repurposed by farmers who nevertheless, Westropp thought, had not built them from scratch. The fort at Derreen survived this fate rather better than most. It is a cashel, meaning a ringfort defined primarily by a stone rather than an earthen boundary, roughly circular in plan, with an internal diameter of around twenty metres. Its stone and earth bank survives to a height of up to one and a half metres on the interior, and a flat-bottomed external fosse, a defensive ditch, remains visible along the eastern arc. Inner facing-stones still stand along the north-east and east sides, and a low break in the bank to the south-west is thought to mark where the original entrance once stood. The fort sits on high pasture with open views to the north-west and north-east, sheltered and partially screened by rising ground to the south-east, a positioning that feels deliberate rather than incidental.

The site was recorded on Ordnance Survey six-inch maps as early as 1842, marked with hachures indicating an earthwork, and appeared again on the 1915 edition. It sits within a wider field system, suggesting that the agricultural landscape around it has a long and layered continuity, the modern pasture grazing the same ground that early medieval farmers once enclosed and worked.