The opening, roughly sixty centimetres wide and only twenty centimetres high, is lintelled, meaning a flat stone spans the gap like a primitive doorframe, and beneath it a drystone passage runs away underground on a roughly north-east to south-west axis. The passage is blocked and inaccessible, but the fact of it is legible enough from the surface.

This is a souterrain, a type of man-made underground structure found widely across early medieval Ireland, typically associated with ringforts. The word comes from the French for "under ground", and these passages were built without mortar, the walls and roof constructed from carefully arranged stone. Their purpose is still debated among archaeologists, with food storage, refuge, and ventilation all proposed as functions. This particular example sits within a rath, which is an earthen ringfort enclosed by an earthen bank and ditch, a form of defended farmstead common in Ireland between roughly the seventh and twelfth centuries. The association between raths and souterrains is well established; many ringforts contain one or more of these subterranean passages, often accessible from inside the enclosed area. Here, the souterrain lies to the west of the rath's centre, within the interior of the enclosure.