What lies here is a souterrain, an underground stone-built passage or chamber constructed in early medieval Ireland, typically for storage, refuge, or both. Most souterrains announce themselves not at all; this one offers only the faintest outline of what was once a deliberate and carefully engineered space beneath the earth.

The souterrain sits in close relationship with the cashel at Longford Demesne, a cashel being a roughly circular stone-walled enclosure used as a defended farmstead during the early medieval period. The positioning is telling. Souterrains were frequently built within or immediately adjacent to such enclosures, and their placement just outside a gap in the cashel wall here suggests a functional connection between the two structures. The two adjoining hollows likely represent the collapsed roof of separate chambers or sections of the underground passage, the stone fill being the remnants of that collapse settling downward over centuries. The single visible lintel, one of the large flat stones that would have formed the roof of the souterrain, is essentially all that remains above the surface to indicate the scale of what was built below.