The entrance has been blocked up, sealing off a three-chambered souterrain, the kind of drystone-lined underground structure built during the early medieval period, most likely to serve as a place of storage or refuge beneath or beside a rath. A rath is a circular earthen enclosure, typically the farmstead of a prosperous family in early medieval Ireland, and this particular souterrain sits within the western sector of one such enclosure.

The structure was described and planned in detail by Knox and Redington in 1916, and their account gives a reasonably precise picture of what lies underground. The first chamber, entered from the south, runs roughly north to south and measures about 3.5 metres long and a metre wide. A simple creep, a low connecting passage requiring a person to crouch or crawl, leads from its northern end into a second chamber, slightly larger at 5.8 metres long and 1.4 metres wide. This second chamber contained an air-vent in its eastern wall, a practical detail that hints at the souterrain being used for more than occasional hiding. From there, a drop-hole creep, a passage involving a downward step or fall rather than a simple horizontal crawl, connects to the third and largest chamber, which runs north-east to south-west and stretches 6.8 metres in length. The whole arrangement, three chambers of increasing size, two different types of connecting creep, and a ventilation feature, suggests a structure built with some deliberate care rather than simply hacked out of the ground.