The roof had collapsed, exposing a small subterranean space that turned out to be one half of a two-chambered souterrain, an underground stone or earth-cut passage built during the early medieval period in Ireland, typically used for storage, refuge, or both.

The structure consists of two earth-cut chambers connected by a narrow creepway, a low linking passage just 0.4 metres high and 0.7 metres wide, which would have required anyone moving between chambers to crawl. The first chamber is roughly circular, measuring about 2.4 metres north to south and 2.2 metres east to west, with a ceiling height of 1.6 metres. Its original entrance, only 0.6 metres wide, had been deliberately infilled at the north-east end, blocking off access from above long before the site was disturbed. The creepway leads south into a second, L-shaped chamber whose main axis runs east to west across 2.5 metres. Also visible within the structure is a blocked construction shaft positioned to the east of the creepway, its outline detectable in the south-east face of the first chamber and the north face of the second. Such shafts were sometimes used during the original building process to remove spoil, then sealed once the work was complete. The deliberate infilling of both the entrance and the construction shaft suggests the souterrain was carefully decommissioned rather than simply abandoned, though the reasons for that decision, and when exactly it happened, remain unknown.