A souterrain is an artificial underground structure, typically built during the early medieval period in Ireland, consisting of one or more chambers or passages constructed from dry-stone walling and roofed with large flat lintels. They are found across the country in their hundreds, sometimes beneath or adjacent to ringforts, and their precise function has long been debated. Likely uses include cold storage for dairy produce, refuge during raids, or simple concealment of valuables. The one at Ballyglass is noted as a monument, but detailed information about its form, dimensions, or condition has not yet been made publicly available.

Without documented specifics for this particular site, what can be said is that souterrains in the west of Ireland tend to follow the same broad tradition as those elsewhere on the island, built by farming communities between roughly the seventh and twelfth centuries. Their survival often depends on whether the land above them has remained undisturbed by deep ploughing or development. Some are entirely collapsed; others retain their roofing stones intact and their passages navigable. The Ballyglass example has at least been identified and assigned a monument record, which means it came to official attention at some point, even if the circumstances of that identification are not currently on public record.