A souterrain, to give it its proper name, is an artificially constructed underground structure, typically built during the early medieval period in Ireland, comprising one or more passages and chambers formed from drystone walling and large roofing slabs. They are found across Ireland in considerable numbers, yet individually they tend to attract little attention, partly because so many remain unexcavated and partly because their precise functions are still debated. Scholars have suggested they served as cool storage chambers, places of refuge, or both, usually associated with a nearby settlement or ringfort that may itself have vanished from the surface record.

The souterrain at Carrowneden is one of those sites that sits quietly in the official record without much accompanying detail having yet been made publicly available. What can be said is that Mayo has a reasonable distribution of these structures, and that the townland name Carrowneden, derived from the Irish, points to a landscape with a long history of habitation. The presence of a souterrain here suggests that early medieval people found this particular corner of Mayo worth settling and worth the considerable effort of constructing an underground chamber, work that would have required both organised labour and a good working knowledge of stone construction without mortar.