The structure in question is a souterrain, a type of man-made underground chamber or tunnel commonly associated with early medieval Irish settlements, built for storage, refuge, or both. What makes this one quietly compelling is that a modern road has apparently been laid directly across it, bisecting the ancient earthwork it belongs to without any particular ceremony.

The souterrain lies within a rath, a circular earthen enclosure of the kind built throughout Ireland during the early medieval period as a farmstead boundary and enclosure. This particular rath at Cormaglava has been further complicated by the addition of a later enclosure or annexe on its eastern side, and it is in that eastern sector, where the two features intersect, that local tradition places the entrance to what people in the area have long called a cave. The passage is said to extend northwards, running beneath the NW-SE roadway that cuts through the northern half of the rath. Above ground, the most visible clue is modest: a low, sod-covered rise of stony ground near the centre of the rath, sitting close to the road. That slight humping of the earth, easy to overlook, may be the only surface indication that something lies underneath.