When the antiquarian Costello surveyed and drew up plans of the structure in 1902, he recorded four chambers connected by three creeps, the low, narrow constrictions that forced anyone moving between chambers to crawl, and which likely served as defensive choke points against intruders. By 1967, Killanin and Duignan could only confirm three. Today, only two chambers are clearly evident, and even those are partly inaccessible, their far ends silted up or collapsed beyond the point where a person could reasonably follow.

A souterrain is an underground stone-lined passage or series of chambers, typically built during the early medieval period in Ireland, often in association with ringforts, the circular enclosed settlements that once formed the basic unit of rural life across the country. Their exact purposes are debated, though storage, refuge, and ventilation have all been proposed. The Ballinphuil example sits inside the bank of a ringfort, and its two surviving chambers are substantial: the first runs roughly north-north-west to south-south-east and measures over eleven and a half metres in length, with a width of close to two metres. The second, positioned just north-north-west of centre and similarly oriented, extends around ten and a half metres. Both are reached now through a gap in the roof rather than any original entrance, and the creeps at their ends, though visible, cannot currently be passed.

What makes the Ballinphuil souterrain quietly melancholy as much as interesting is this steady recession of knowledge. Each generation of observers has found slightly less than the last, and the gap between Costello's four chambers and the two that remain open today reflects not just physical collapse but the gradual erasure of a structure that once had clear internal logic, a sequence of spaces linked by deliberate design. The creeps that blocked further exploration during survey work are a small but pointed reminder that the underground can be uncooperative even when it survives.