It is a souterrain, a type of stone-lined underground tunnel or chamber that was commonly constructed during the early medieval period in Ireland, typically between the seventh and twelfth centuries. These structures were built in association with farmsteads and ringforts, and served various purposes depending on the site: cool storage for dairy produce, places of refuge during raids, or simply as drainage systems. That one exists at Tearmann Caithreach is known; the details of its construction, its current state, and its precise relationship to any surrounding settlement remain, for now, opaque.

The place name itself carries some interest. Tearmann, derived from the Latin terminus by way of Old Irish, generally refers to sanctuary land associated with an early church or monastic site, ground that was set apart and afforded legal protection in early Christian Ireland. Caithreach can suggest a walled or enclosed settlement. Whether this souterrain was connected to such a religious or ecclesiastical enclosure, or whether the name simply echoes a long-gone institutional landscape, is not clear from what is currently available. Mayo has a dense distribution of souterrains, many of them found in areas with strong early medieval settlement, and the county's western seaboard was an active zone of monastic and lay activity from the sixth century onwards. The co-occurrence of a souterrain with a townland name suggesting sanctuary land is the kind of detail that tends to reward closer investigation, even if that investigation has yet to be fully carried out or made public.