A souterrain, to use the proper term, is an artificial tunnel or chamber built into the earth, typically during the early medieval period in Ireland, between roughly the sixth and twelfth centuries. They were constructed from dry-stone walling and roofed with large flat lintels, then covered over with soil. Depending on the site, they served as storage spaces, refuges, or both, and they are found scattered across Ireland in considerable numbers, though many remain poorly understood and some have never been properly excavated.

The souterrain at Graffy is one of those sites where the record falls quiet. Its presence on the landscape is noted, but the details that would bring it into sharper focus, its dimensions, its condition, whether it retains any structural integrity, what lies around it, have not been made publicly available. What can be said is that County Mayo contains a number of such monuments, often associated with early Christian settlement or with the enclosed farmsteads known as raths or ring-forts. The presence of a souterrain in Graffy suggests, at minimum, that someone in the early medieval period chose this particular patch of ground as a place worth defending or provisioning underground, and that enough of whatever they built has survived to be recorded.