The opening is modest, almost dismissible, set about nine metres inside the northern edge of the enclosure. What lies beyond it, however, is a carefully engineered underground structure of a kind that would have served early medieval communities as a place of refuge, food storage, or concealment. A souterrain, to use the proper term, is an artificially constructed underground passage or series of chambers, typically associated with ringforts across Ireland and built without mortar, relying instead on the weight and placement of stone.

The most detailed account of this particular souterrain comes from a report by W. Fitzgerald, Sergeant of the Garda Síochána, dated 21st August 1930, preserved in an OPW correspondence file. His description is admirably precise. The entrance drops about 1.8 metres vertically, then narrows into a low built-stone passage roughly 0.9 metres high and only 0.6 metres wide, extending about 1.2 metres before opening into the first of three chambers. That first chamber is egg-shaped, constructed from dry-stone walling that inclines inward as it rises, corbel-fashion, and is capped with large flat flags. It measures approximately 2.4 metres by 1.8 metres, with a height of around 1.5 metres. A second passage leads from it into a similarly sized chamber, and from there another passage continues into a third, somewhat larger chamber. At the point Fitzgerald made his inspection, the third chamber had partially collapsed, with fallen earth blocking any further exploration. The structure as a whole is a good example of the architectural care that went into souterrain construction, the corbelled walls and flagstone roofing representing a considered solution to underground stability using only locally available stone.