The opening measures roughly 45 centimetres east to west and just 20 centimetres north to south, yet it gives onto a subterranean passage a metre high and 65 centimetres wide, built from dry-laid stone and roofed with flat lintels. It is a souterrain, the term used for the underground passages and chambers that appear at many early medieval Irish sites, most likely used for storage, refuge, or both. This one, however, is partly blocked with stones and cannot be entered.

What makes the Barnacahoge souterrain particularly worth noting is its setting. It sits roughly at the centre of a cashel, the kind of circular stone-walled enclosure that served as a farmstead or minor fortified residence in early medieval Ireland. That cashel has been destroyed, its walls reduced to little more than a trace in the landscape, but its outline was recorded and the souterrain within it survives, at least in part. The passage runs north to south underground, and about eight metres to the south a stone slab protrudes from the grass, which may mark the continuation or southern extent of the same passage. The two features together, the tight opening and the distant slab, suggest a structure longer than what is immediately visible at the surface.