That hollow is all that visibly remains of a souterrain, an underground stone-lined passage, typically built during the early medieval period, that would originally have served the ringfort's inhabitants as a place of refuge, storage, or both. These structures were carefully constructed affairs, often featuring multiple chambers connected by low, narrow gaps called creepholes, through which a person had to crawl to pass from one section to the next, a design that made pursuit by an intruder awkward and dangerous.

When O'Cuileanain examined the site in 1943, the souterrain was already in a state of partial collapse, though enough remained to identify several chambers and creepholes intact. By 1970, further deterioration had made the structure unstable enough that the decision was taken to fill it in, a pragmatic measure to prevent injury but one that effectively sealed off whatever remained underground. McCarthy, writing in 1977, recorded that collapse and the subsequent filling. The ringfort within which the souterrain sits, a circular enclosure of the kind that once dotted the Irish countryside in their thousands, survives as the broader context for this vanished underground feature, the depression at its centre acting as a quiet indicator of what lies below.