This is the nature of a souterrain, an underground structure of the early medieval period, typically stone-lined or earth-cut and associated with nearby ringforts. They are thought to have served as places of refuge, cold storage, or concealment, and the one at Ardahill was among the more elaborately arranged examples of its kind: not a simple tunnel but a linked sequence of corridors designed to disorient or slow an intruder.

The souterrain came to light in 1964, and what investigators found was a system of four connected passages arranged in a zig-zag formation, leading to a northeast-southwest chamber roughly three feet high and five feet wide. The whole thing had been cut directly into the clay subsoil rather than built from stone, which made it both harder to detect and more vulnerable to collapse over time. The chamber itself was already partially choked with debris when it was examined, the material spreading inward from its southwestern end, though what remained visible matched the passages closely in its modest dimensions. It sits within what may have been a rath, the Irish term for a ringfort, a circular enclosed settlement of the early medieval period, and the combination of enclosed settlement above ground and concealed passages below is a familiar one across the Irish landscape, though no less intriguing for that.

Nothing of the souterrain is visible at the surface today. The clay through which it was cut offers no lasting outline, and whatever earthworks might once have defined the associated rath have left no obvious trace either. The site exists now mainly as a coordinate on a map and a description in the record, a ghost of a structure that was already half-buried in its own debris when it was first formally noticed.