The entrance is easy to miss, and the interior, barely 0.8 metres in height, is largely blocked up now, but what remains is recognisably the work of careful hands: drystone masonry walls, built without mortar, and a roof of flat-laid slabs laid across the top to create a covered underground space.

This is a souterrain, a type of man-made underground structure found widely across early medieval Ireland, typically associated with ringforts or raths. A rath is a circular enclosure defined by one or more earthen banks and ditches, and was the most common form of rural settlement in Ireland during the early medieval period, roughly from the fifth to the twelfth century. Souterrains were built within or beneath such settlements, their precise function still debated among archaeologists. The most widely accepted view is that they served as places of refuge or concealment, and possibly as cool storage for perishables. The one at Cabraghkeel follows the familiar construction method: drystone-lined walls and a corbelled or slab roof, all built to human scale, if a cramped one. The passage here is associated with the adjoining rath, and together the two features represent the kind of layered, quietly complex site that turns up in fields across the Irish countryside with little ceremony or signage.