A souterrain, as these structures are known, is an underground passage or chamber built in dry stone, typically associated with Early Medieval ringforts, and used variously for storage, refuge, or as a place of concealment. The Cloghardeen example was unusual in the quality of its detail: the chambers were roughly circular, built from uncoursed limestone fitted together without mortar, and each one was capped at the apex of its beehive vault by a single flat stone. Most striking of all, each chamber had been fitted with two ventilation shafts near the top, angled in opposite directions toward the surface, a feature considered noteworthy even at the time of its description.

The Reverend James Graves recorded this souterrain in some detail around 1870 to 1871, at which point the structure had already been cleared out. The finds recovered during that clearance were telling: animal bones from cattle, pig, sheep, deer, and goat, quantities of sling stones, bone pins, fragments of charcoal, and a carved bone. The mix of domestic refuse and possible defensive materials, the sling stones in particular, fills in something of the human activity that once took place in these close, low-ceilinged spaces. The passages themselves were square-headed and roofed with single spanning stones, the walls fitted together without any regular coursing, the stone faces left roughly as they had come from the quarry.

Today the entrance survives as a hollow in the ground, the opening itself covered over. It is a modest trace of what Graves once described in enough detail to let a reader feel, almost physically, the experience of crawling from one dim chamber into the next.