A souterrain, the term used for these dry-stone lined underground tunnels built throughout early medieval Ireland, typically served as storage space or a refuge, and this one at Caherpierce keeps its purpose quietly to itself. There is no ringfort crowning the hillside above it, no church nearby, nothing to frame it in the conventional way. It simply exists in the landscape, associated only loosely with the faint remains of human activity nearby.

The structure is roughly L-shaped, comprising two passages. The first runs about 1.9 metres south-south-west from a pit entrance before angling sharply to continue west-north-west for a further 3.65 metres. The entrance pit itself is roughly a metre square, and the accessible passage opening is a tight thing, just 38 centimetres wide and 50 centimetres high, roofed by a single flat slab. Inside, the walls are of dry-stone construction throughout, inclining slightly inwards to meet a roof of large flat slabs, with smaller stones plugging the gaps and occasional corbels, projecting stones used to support the roofing slabs from below, bearing some of the weight. The passage height varies between half a metre and just over a metre, partly because debris has accumulated near the openings over time. Roughly a metre from the far end of the second passage, part of a side wall has collapsed, and what appears to be a deliberately excavated hollow to the south-west now offers the easiest way in. Whether a section of rebuilt walling nearby is the work of an early medieval hand or a more recent one remains unknown. Around 100 metres to the south, the foundations of a circular stone hut survive, and two small enclosures lie in the adjacent field to the west, along with disused field walls stretching east into Killeenagh townland, some of them absent from the Ordnance Survey maps. Taken together, these fragments suggest a small, dispersed settlement of some kind, though the souterrain itself provides no clear date or owner.