Beneath the surface lies a souterrain, an underground passage or chamber constructed during the early medieval period, typically cut into the earth and lined or roofed with stone. These structures are found across Ireland in association with ringforts and are thought to have served as places of refuge, cool storage, or concealment. At Glanbannoo, the subsidence of the ground itself is what betrays the feature, the earth having settled or collapsed slightly over the centuries into the void below.

The ringfort here, recorded alongside this souterrain, forms a paired site that would have been the homestead of a farming family of some local standing during the early medieval period. A ringfort is essentially a circular enclosure, defined by one or more earthen banks and ditches, that served as both a domestic and defensive boundary. The souterrain at Glanbannoo was observed by Myler in 1998, who noted that its entrance lay on the north external bank of the ringfort, a position that would have placed it just outside the main enclosure but still within a defensible zone.