A souterrain is an underground stone-lined passage or chamber, typically constructed during the early medieval period in Ireland, most often associated with settlement sites and thought to have served for storage, refuge, or both. That one exists on an island this remote makes it quietly remarkable; these structures are labour-intensive to build, and their presence on a small offshore island suggests a community that was both settled enough and motivated enough to construct one.

Beyond the bare fact of its existence, the documentary record for this particular site is, at present, thin. What can be said with confidence is that souterrains in the Irish context are generally dated to the period roughly between the seventh and twelfth centuries, and they are frequently found in association with ringforts or early ecclesiastical enclosures. Islands off the Mayo coast were not uninhabited in that era; monastic and secular communities alike made use of coastal and island locations, and the effort of island life did not preclude the kind of organised construction that a souterrain represents. The name Beetle North Island is itself unusual, and the cluster of small islands in this part of Clew Bay and the broader Mayo coastline has its own layered history of habitation, fishing, and seasonal use stretching back millennia.