It is a modest structure in physical terms, but its survival intact into the twenty-first century, unrecorded and unremarked, is itself quietly remarkable.

A souterrain is a deliberately constructed underground chamber or series of passages, typically built during the early medieval period in Ireland and associated with nearby settlement sites. They were lined and roofed with stone, and their precise function continues to be debated, though cold storage and refuge are the explanations most commonly advanced. The example at An Baile Riabhach is stone-built, running roughly NW-SE along a passage before turning to the NNE into a single chamber, giving it an L-shaped plan overall. Its total length is 7.5 metres, according to information provided by M. Connolly.