Two blocked openings, spaced roughly four metres apart, are the only outward signs that something deliberate and carefully built sits just below the surface. This is a souterrain, an underground stone-built passage or chamber constructed in early medieval Ireland, typically used for storage, refuge, or as a cool space for dairy produce. Both entrances are now stopped up with stones, and the interior has not been formally accessed in recent times.

The structure sits in the south-western quadrant of a cashel, a type of early medieval stone-walled enclosure, generally circular or oval in plan, which served as a farmstead or small settlement. The combination of cashel and souterrain is not unusual in the Irish archaeological record; the two features often occur together, with the souterrain providing a concealed underground space within or close to the protected area of the enclosure. At Barnalyra, the souterrain's presence is indicated above ground by a low, roughly rectangular stony rise measuring approximately 3.8 metres on its north-west to south-east axis and 5.8 metres on its north-east to south-west axis. Local knowledge holds that it is built in drystone construction with lintel roofing, meaning flat stones laid horizontally across upright side walls to form a ceiling, without the use of mortar.