A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval ringforts, and used variously for storage, refuge, or ventilation of adjacent structures. At Mossgrove, the ground gives nothing away. There is no hollow in the turf, no slight depression after rain, no exposed stonework at a field edge. Whatever lies below, if anything does, has been sealed entirely from view.

The souterrain is said to be connected to a nearby ringfort, a type of enclosed farmstead common across Ireland during the early medieval period, roughly the fifth to twelfth centuries, consisting of a circular area bounded by one or more earthen banks and ditches. The association between ringforts and souterrains is well established elsewhere in Cork and across the country, and the combination would have made practical sense, with the underground passage providing a concealed space directly accessible from within the enclosure. At Mossgrove, however, the tradition rests on local memory rather than any confirmed physical evidence. No excavation appears to have established the souterrain's dimensions, construction, or precise location.