It is a souterrain, a type of structure built during the early medieval period, typically between the sixth and twelfth centuries, consisting of one or more chambers and connecting tunnels constructed from dry-stone walling and roofed with large capstones. Their exact purpose has long been debated, with theories ranging from food storage and refuge to ritual use, though the practical answer likely varied from site to site.

Souterrains are found in considerable numbers across Ireland, particularly in the northern and western counties, often in association with ringforts or early ecclesiastical settlements. The townland name Carrowleagh derives from the Irish, broadly meaning a grey or stony quarter-land, which is itself a unit of land division inherited from Gaelic administrative tradition. Mayo contains a significant concentration of such underground structures, many of which have never been formally excavated and survive only as surface depressions or fieldwork records. Without more detailed documentation having entered the public record for this particular example, the specifics of its construction, dimensions, or condition remain difficult to establish with any confidence.