It is a souterrain, a type of man-made subterranean structure built during the early medieval period, typically constructed from dry-stone walling and roofed with large capstones. These passages were most commonly used for cold storage or as places of refuge, and they appear across Ireland in considerable numbers, though many remain poorly documented or entirely unknown to anyone outside the immediate locality.

The details of this particular example, its dimensions, construction method, condition, and any associated surface features, have not yet been made available. What can be said is that souterrains in the west of Ireland frequently occur in association with ringforts, the circular enclosed farmsteads that dominated the rural landscape between roughly the sixth and twelfth centuries. The presence of one in Cloonygowan hints at a settled, functioning community in this part of Mayo during that period, people who invested real effort in cutting and lining an underground chamber, suggesting the site was used for more than a single season.