Two subtle depressions sit in the southern half of a ringfort, and at least one of them has given something away: a flagstone, exposed at the surface, hinting at a roofed underground passage that has since fallen in on itself. This is a souterrain, or what remains of one. Souterrains are dry-stone or lintelled underground chambers and tunnels, typically associated with early medieval settlement in Ireland, and thought to have served for storage, refuge, or both. Most are known only because the land above them has begun to sink.

The ringfort with which this souterrain is associated is a separate, numbered site in its own right. Ringforts, the circular enclosures defined by earthen banks or stone walls that dot the Irish countryside in their thousands, were the farmsteads of early medieval Ireland, occupied roughly between the fifth and twelfth centuries. It was common practice to cut a souterrain beneath or beside the living area, accessible from inside the enclosure. At Horsemount, the two depressions, one to the south-west and one to the south-east of the interior, may correspond to distinct collapsed chambers, suggesting the underground structure was not simply a single passage but something more complex in layout. The exposed flagstone in the south-western depression is the only element now visible, a fragment of roofing that once sealed a space people used and moved through.