A souterrain is an underground passage or chamber, typically constructed from stone, built during the early medieval period as a place of refuge, food storage, or concealment. They are found in their hundreds across Ireland, yet each one represents a particular community's decision, at a particular moment, to dig down and create a hidden space beneath the surface of daily life.

Beyond the confirmed fact of its existence as a recorded monument at Roslahan, the specific history of this souterrain remains largely undocumented in the public domain. What can be said is that souterrains of this type are generally associated with early Christian period settlement, roughly between the sixth and twelfth centuries, and were often constructed in association with raths or ringforts, the enclosed farmsteads that once organised rural life across much of the country. Mayo's landscape contains a considerable number of such features, many of them poorly preserved or visible only as cropmarks and soil disturbances from the air. That a souterrain survives at Roslahan as a formally recognised monument places it within a category of archaeological sites considered significant enough to warrant legal protection, even where detailed investigation has not yet been published.