A cashel is a type of ringfort built from stone rather than earthen banks, typically dating to the early medieval period, when such enclosures served as farmsteads and household compounds for local farming families. This one at Derreen is poorly preserved, its defining bank reduced in most places to a width of between one and a half and just over two metres, with the outer facing surviving to a maximum height of only 0.7 metres along the southern to northern arc. No trace of an original entrance remains visible, which is one of the more disorienting qualities of the site: a walled enclosure, apparently complete, with no obvious way in or out.

The structure was recorded on Ordnance Survey six-inch maps as far back as 1842, marked with hachures indicating an earthwork or enclosure, and it appeared again on the 1915 edition. Its interior measures roughly 29.7 metres north to south and 21.8 metres east to west, making it a reasonably substantial enclosure. A bank extending westward from the north-western edge may be a remnant of a contemporary field system associated with the cashel, suggesting that what survives today is only part of a wider agricultural landscape that once surrounded the site. Complicating the picture further, a later field wall runs along the enclosure's edge from the north-east to the south-south-east, and a modern livestock pen occupies the north-eastern corner, layering several centuries of land use onto the same modest patch of hillside. The result is a site where early medieval stonework, nineteenth-century boundary improvements, and contemporary farming all occupy the same ground simultaneously.