The opening measures just forty-five centimetres wide and thirty centimetres high, which gives some sense of the intent behind its construction. This was not built for convenience.

The structure is a souterrain, an underground stone-built passage of early medieval Irish origin, typically associated with ringforts and cashels, and thought to have served as a place of concealment, cold storage, or refuge. A cashel is a stone-walled enclosure, the drystone equivalent of an earthen ringfort, and this souterrain sits within one. The passage runs in an L-shape for approximately ten and a half metres in total. The first section stretches roughly four and a half metres from east to west, open to the sky and relatively shallow at half a metre deep. At its eastern end the passage makes a right-angle turn northward for at least five metres, where the character of the structure changes: here the passage is roofed with stone lintels, creating a covered corridor. The small opening at the southern end of this roofed section, with its single wide roof lintel, would have made entry difficult and slow, which was presumably the point. Anyone attempting to force their way in would have been thoroughly disadvantaged.