A souterrain, for those unfamiliar with the term, is a man-made underground structure, typically dry-stone built, associated with early medieval settlement in Ireland and thought to have served as a place of refuge, cool storage, or concealment. What makes the one at Cluidrevagh quietly remarkable is that it has come to light not through any planned excavation, but twice by accident, and still has not been fully explored.

The structure was first noticed around twenty years before it was formally recorded, when construction of a nearby bungalow brought a bulldozer into contact with some of its roofing lintels. The resulting opening was back-filled rather than investigated, and the site returned to obscurity. It was only during later landscape gardening that further lintels were disturbed, revealing a dry-stone chamber running roughly northwest to southeast, measuring just over five metres in length and tapering slightly from around 1.46 metres wide at the northwest end to 1.25 metres at the southeast. The southeast end curves inward, suggesting an original end-wall still more or less intact. A step in the floor separates the lower northwest portion of the chamber from the slightly higher southeast section. At the northwest end, the old back-fill from the first discovery remains in place, and the shape of the structure strongly suggests that the passage continues beyond that blockage, its full extent still unknown.