This is a souterrain, an artificial underground structure built in early medieval Ireland, typically of drystone construction and covered with large flat lintels. Such passages were associated with ringforts and cashels, and were used variously for storage, refuge, or both. What makes the Ballyganner example quietly interesting is that its route is not straightforward: after a short initial section heading east, a large upright slab protruding from the north wall forces any traveller to turn south, then east again, as though the builders went out of their way to prevent easy movement through it.

The souterrain sits within the western interior of a cashel, the term for a stone-walled circular enclosure of the early medieval period, the Irish equivalent of an earthen ringfort. The surface evidence today is a rubble mound roughly 12.5 metres east to west and between four and five metres wide, rising to about half a metre in height. Along the top of this mound, the upper courses of drystone walling are just traceable. The entrance, a narrow opening of around 0.7 metres wide, appears to be on the north side of the western end. Inside, the passage runs to a minimum measurable length of about 6.2 metres, widening to 1.6 metres in its eastern section before gradually narrowing to 1.2 metres. The eastern end is blocked by a fallen lintel, though the structure may extend a further two metres or so beneath the rubble beyond. The passage is partly filled with loose stones and cracked lintels, leaving the full extent uncertain.