The enclosure measures roughly nine metres east to west, with a straight western side running 7.2 metres and walls that, though largely reduced to low stony ridges, retain a height of up to 0.3 metres in places. A northeast-facing entrance breaks the circuit. What gives this modest structure its quiet interest is not the enclosure itself in isolation, but what surrounds it: the remains of a wider field system, fragments of which abut the enclosure's northwest corner and run along its north-northeast wall.

Relict field systems, the ghost outlines of ancient agricultural organisation preserved beneath blanket bog, are among the more arresting phenomena in the Irish upland landscape. Peat growth, which began in many areas after the climate deteriorated in the later prehistoric period, effectively sealed and protected the walls and boundaries beneath it. At Derreenacahill, sections of that broader field system survive alongside the enclosure, suggesting this hillside was once a worked and organised place, divided, bounded, and entered through a deliberate gap. Drystone construction, using unmortared stone laid carefully to hold by weight and friction alone, was the standard technique for such boundaries across centuries of Irish farming. The enclosure's D-shape, defined by a curving northern arc and a flat western side, is a form with deep roots in Irish archaeology, appearing in contexts ranging from early medieval settlement to prehistoric land management, though the notes for this site do not assign a specific date.