The structure is a souterrain, a type of artificially constructed underground passage built from dry, uncemented stone, typically associated with early medieval settlement and used variously for storage, refuge, or concealment. What makes this one quietly remarkable is its scale and complexity: three chambers, connected by low crawl-ways known as creeps, with at least one passage still blocked by collapsed soil and the full extent of another sealed beneath a sloping heap of rubble.

The souterrain sits inside a rath, an enclosed circular settlement defined by an earthen bank and internal scarp, and the underground structure was recorded as early as 1878 by O'Rorke, who described it as a cave ten or twelve feet long and four or five feet wide, roofed with flags laid across uncemented stone walls. What O'Rorke saw was only a fraction of what survives. Subsequent inspection revealed two intact interconnecting chambers aligned roughly north to south. Chamber 1, the more southerly of the two, is just over five metres long, nearly two metres high, and built from random rubble with walls that incline inward to support a roof of massive overlapping slabs. An air vent was noted in the south-west wall. From chamber 1, two creeps open at ground level: low, lintel-roofed passages that require crawling to navigate. One leads north into chamber 2, which extends for at least ten metres and stands 2.2 metres at its highest, though it narrows towards a blocked northern end. The second creep, only 0.71 metres wide and 0.62 metres high at its entrance, heads north-west but is choked by collapsed soil. A third chamber, now roofless and engulfed in overgrowth to the west, may have been the original entrance passage to the whole system. During a 1994 inspection, a large stone slab was noted protruding from the ground about three metres east of this ruined chamber, hinting at yet another passage or chamber still unexcavated beneath the surface.

Access to the surviving underground chambers is currently through an unintended opening, roughly 45 to 50 centimetres across, caused by the collapse of a roof lintel above chamber 1. The creep connecting chambers 1 and 2 narrows to just 54 centimetres wide at its tightest point before widening again, and the passage into the second creep is barely large enough to enter at all. Chamber 3, at the surface to the west, is partly filled with soil and stone and heavily overgrown, and may yet conceal a connecting passage to the chambers below.