Look more carefully, however, and a pattern emerges: the slabs are evenly spaced, deliberately arranged, and they mark the roof of something entirely underground. Beneath them runs a souterrain, an artificial underground passage built in the early medieval period, typically used for cool storage or as a refuge in times of danger. At Ballycahan, the structure curves gently from northeast to southwest across a length of eleven metres and a width of 1.6 metres, its drystone walls, built without mortar to a depth of 0.7 metres, just visible in the gaps where the limestone lintels do not quite meet.

The souterrain does not sit in isolation. It runs along the northwestern perimeter of an enclosure and within the northern extent of a cashel, a type of stone-walled ringfort common across the west of Ireland. Cashels varied considerably in complexity, and this one at Ballycahan is no exception: a second, smaller enclosure lies roughly eleven metres to the southwest, contained within the same larger cashel boundary. The relationship between the souterrain and these nested enclosures suggests a settlement of some organisation, where underground access was deliberately positioned relative to the structures above ground, though the precise chronology of how these elements related to one another remains unresolved.