Three capstones, resting on large corbels, break the surface near the north-western edge of an old cashel, hinting at a chamber that stretches some 6.8 metres beneath the earth. The rest is infilled and out of reach, but those flat stones protruding from the soil are enough to identify what lies below.

The structure is a souterrain, an underground passage or chamber typically built during the early medieval period in Ireland, often associated with nearby ringforts or enclosures and thought to have served as storage space, a place of refuge, or both. Here, the souterrain adjoins the bank of a cashel, which is a stone-walled circular enclosure of the kind common across the west of Ireland, particularly in areas where field stone was plentiful and timber was not. The two features together suggest a settlement of some organisation and permanence, though the archaeological record for Caherduff offers little more than the physical dimensions: a stone-built chamber of considerable length, sealed and silent.