There is no mound, no hollow, no depression in the turf to suggest that anything lies beneath. The ground simply looks like ground.

A souterrain is an artificial underground structure, typically earth-cut or stone-lined, built during the early medieval period in Ireland, most commonly associated with nearby ringforts and used for storage, shelter, or refuge. The Cullenagh example, documented by Twohig in 1976, consists of two sub-rectangular chambers cut directly into the earth. The first chamber measures 4.5 metres long, 1.3 metres wide, and 1 metre high; the second is slightly wider at 1.8 metres, roughly the same length, and the same low height. Both are connected by a creepway, a narrow passage requiring a person to crawl through, which was a deliberate design feature rather than a limitation of construction. Each chamber also has a construction shaft, one at the north-east corner of the first chamber and one at the south-east corner of the second, openings used during the building process and afterwards sealed. Perhaps most intriguing is a blocked-up creepway on the eastern side of the second chamber, which may represent the original entrance to the whole system, later deliberately closed off. The sequence of modifications suggests the structure was adapted over time, its access points reconsidered and rearranged by whoever used it.