The feature is a souterrain, an artificial underground structure of the early medieval period, typically built from drystone walling and used for storage, refuge, or concealment. Most survive only as collapses and crop marks, and the one at Loughaunbrean is no exception.

The souterrain sits within the northwest quadrant of the ringfort recorded at this site, a ringfort being an enclosed farmstead of roughly the early medieval period, usually defined by one or more earthen banks and ditches. The underground chamber itself, where its remains can still be traced, measures approximately 3.5 metres in length and 1.6 metres in width, built in the drystone technique where stones are laid without mortar. The passage connecting it to the surface has largely disappeared, leaving only the shallow ground depression as evidence of its original course. How the souterrain related to the daily life of whoever occupied the ringfort above it, whether it held dairy produce in the cool dark or offered a place to shelter during a raid, is not recorded, and the site itself is too far gone to say with certainty.