The presence of a souterrain, an underground stone-lined tunnel or chamber typically used for storage or refuge in early medieval Ireland, sets this site apart from the many plainer ringforts scattered across the Irish countryside. It is an easy detail to miss entirely from the surface, where the vegetation has done its best to reclaim whatever geometry remains.

The rath at Cahernabrock sits in low-lying pasture and takes a roughly circular form, measuring 43 metres north to south. Two earthen banks define its boundary, separated by a fosse, the ditch that runs between them and whose spoil once helped raise the banks on either side. The inner bank survives to about 0.8 metres in height, though both it and the fosse are heavily overgrown. The outer bank, somewhat lower at 0.5 metres, retains what appears to be an original entrance on its eastern side. The souterrain recorded in the interior adds a layer of complexity to what might otherwise read as a modest, unremarkable enclosure. A ringfort of this double-banked type, sometimes called a bivallate rath, would have offered its occupants a degree of additional security beyond the single-bank norm, suggesting a settlement of some local consequence in the early medieval period, roughly between the fifth and twelfth centuries. The site and its associated souterrain were catalogued as part of a 1994 archaeological survey of the Ballinrobe district, compiled by D. Lavelle and covering the wider Lough Mask and Lough Carra area.