The site is a rath, or ringfort, the most common monument type in the Irish landscape, built during the early medieval period as a farmstead enclosed by earthen banks and ditches. What makes the one at Derrylough quietly interesting is precisely how little of it remains, and how much that absence tells.

The enclosure is oval, measuring roughly 37 metres on its longer north-west to south-east axis and about 30 metres across. Today it survives mainly as a scarp, a low, abrupt slope in the ground, rising somewhere between 0.4 and 0.5 metres. Faint traces of what was once a bank can still be made out along its edge. A fosse, the defensive ditch that typically ran between an inner bank and an outer one, was noted in a survey report from 1976, as was the outer bank itself, but neither is any longer visible at ground level. The site has been quietly levelling out, season by season, under the weight of ordinary agricultural use. One feature does survive with more conviction: a single stone set on its edge at the south-east of the enclosure, measuring just over a metre in length. Its position suggests it may once have formed part of an entranceway, a threshold that people and animals passed through daily during the centuries when this enclosure was a working farm rather than a slight rise in a field.