These underground stone-lined passages, built during the early medieval period, were constructed by hand from dry-laid or mortared stone and covered with large capstones, then buried beneath the earth. Their exact purpose remains debated; they may have served as cold storage, places of refuge, or both, and they are found in considerable numbers across Ireland, often associated with ringforts or early Christian settlements above ground. The one at An Caiseal is recorded as a monument, which places it in the company of hundreds of similar structures that survive, fully or partially, across the country.

Beyond its classification and location, the specific details of this particular souterrain, its dimensions, its condition, whether it has been excavated or examined, and what if anything was found within it, are not currently available in the public record. That absence is itself a small curiosity. Many such structures across Mayo and the west of Ireland have never been formally excavated, surviving instead as grassy depressions or low earthwork features that only a trained eye would distinguish from a natural hollow in the ground. An Caiseal, meaning "the stone fort" or "the cashel" in Irish, is a placename that suggests the wider archaeological landscape here may once have included a cashel, a type of stone-walled enclosure typical of early medieval Ireland, of which the souterrain might have been a component part.