This is the souterrain, an underground structure of the kind commonly built by early Irish farming communities, typically used for cool storage or as a place of refuge. It sits at the centre of a ringfort, a subcircular enclosure of the sort that once dotted the Irish landscape in its thousands, home to a single farmstead and its household. What makes the Derryleagh example quietly compelling is that it does not appear on Ordnance Survey maps at all, occupying a kind of official absence in the landscape despite being physically present, if battered, on the ground.

The enclosure measures roughly 18.5 metres north to south and 21 metres east to west internally, though much of what once formed its boundary wall has collapsed into a spread of loose stone across the circuit. The western sector survives best, where a short stretch of rough stone facing is still visible and the wall reaches a maximum surviving height of about 0.9 metres. The entrance to the souterrain is set centrally within the raised interior. It is a notably tight opening, only 0.6 metres by 0.5 metres, leading into a lintelled passage running north to south for 2 metres, its sides formed of earth rather than stone. At the southern end a narrow creepway connects to a second passage, 4 metres long, which drops in height from 1.3 metres at its northern end to less than half a metre at its southern tip. The whole structure is roofed with flat lintels and retains its earth side walls. The site was documented by archaeologists A. O'Sullivan and J. Sheehan as part of their survey of the Iveragh Peninsula, published by Cork University Press in 1996.