A souterrain, to use the term for these deliberately constructed subterranean chambers, is essentially a man-made tunnel or series of chambers built from dry stone, typically during the early medieval period in Ireland. They were dug beneath or beside settlement sites, most likely for food storage, refuge, or both, their cool dark interiors keeping dairy produce fresh while also offering a place to hide in times of threat. The one at Erriff is recorded as a monument, but beyond its existence and location, almost nothing has been made publicly available about it.

That silence is itself telling. Mayo is rich in early medieval archaeology, and souterrains turn up across the county in association with raths, the circular earthen enclosures that once enclosed farmsteads across the Irish countryside. The Erriff valley, running inland from the head of Killary Harbour toward the Partry Mountains, is a landscape shaped as much by glacial action as by human habitation, and it is exactly the kind of remote, marginal ground where evidence of early settlement can survive simply because later development never disturbed it. Without more detailed records in the public domain, the date of construction, the form of the passages, and any associated features remain unknown.