A souterrain, to use the term that archaeologists apply to these structures, is an artificial underground chamber or series of chambers, typically constructed during the early medieval period in Ireland. They were built by hand, usually from dry-stone walling roofed with large lintels, and then covered over with earth. Their precise function is still debated; they may have served as cool stores for dairy produce, as places of refuge, or as a combination of both. What makes their presence quietly arresting is how little they announce themselves above ground, even now.

Souterrains are found across Ireland in considerable numbers, often associated with ringforts, the circular enclosed farmsteads that were the dominant settlement type of early medieval Irish society, roughly from the fifth to the twelfth centuries. Mayo has its share of both. The Shanwar example is recorded as a named monument, which places it within a category of archaeological site considered significant enough to warrant formal identification, even if the documentary record around it remains thin. Beyond its location in the townland of Shanwar, the specific details of its construction, dimensions, and any associated surface features are not currently available in the public record.