A souterrain is an underground stone-lined passage, typically associated with early medieval settlement and used variously for storage, refuge, or both. This one was so completely stripped of its roofing slabs and packed with fill that only the accidental removal of the stone heap above it revealed two upright slabs at the bottom, the last visible hint of an entrance passage that had otherwise vanished entirely.

The discovery came in 1928, when archaeologists R. A. S. Macalister and R. Lloyd Praeger excavated the site. Lifting the stones, they found soil that had turned unusually black against the normally red iron-oxide earth, suggesting a long accumulation of decayed organic material, though relatively few bones. Following the clue of those two edge-set slabs, they uncovered a straight passage running northward for approximately 8.5 metres, narrowing to just 0.38 metres at its southern entrance before gradually widening to 0.78 metres, then opening into an oval beehive chamber roughly 1.5 metres long and just over a metre across at its widest point. The floor drops some 0.71 metres over its length, reaching nearly 1.5 metres below the surface at its deepest. Wall traces on the eastern side suggested the original structure may have extended to around 11.27 metres in total. The enclosure containing it is part of a dense cluster of monuments on Uisneach, a site of considerable ceremonial significance in early Irish tradition, surrounded by barrows, earthworks, a ringfort, a holy well, an ancient embanked roadway, and Logh Lugh, a ritual pond lying approximately 170 metres to the north-west. The 1837 Ordnance Survey six-inch map already marked a 'Cave' at the centre of the enclosure, and also depicted what appear to be hut foundations within the ringfort interior and its annexe. Rache Schot's re-examination of Macalister and Praeger's findings in 2006 has since offered a revised interpretation of what the excavation data actually shows about this complex of structures.