It is a souterrain, a type of structure built during the early medieval period in Ireland, typically consisting of one or more dry-stone tunnels or chambers excavated into the earth or constructed and then covered over. Their precise function has long been debated, with theories ranging from food storage and refuge to ritual use, though most archaeologists now lean towards a combination of cool storage and occasional shelter in times of threat.

Souterrains are found across Ireland in considerable numbers, often associated with ringforts or early ecclesiastical settlements, suggesting they were a standard feature of rural life between roughly the sixth and twelfth centuries. The placename Rathgranagher itself is suggestive: "rath" points to a ringfort, a circular earthwork enclosure that was the most common form of farmstead in early medieval Ireland, and it would be entirely typical for such a site to have a souterrain attached. Beyond its existence and location in Mayo, the specific details of this particular structure, its dimensions, the number of chambers, its current condition, and any associated finds, remain unavailable in the open record at this time.