The souterrain sits in the north-west quadrant of a cashel, a type of stone-walled ringfort, and stretches eight metres along an east-west axis. What is visible above ground is already enough to prompt a second look: a run of exposed parallel stone lintels, each around 1.7 metres in length, lying in a row like the spine of something long buried.

Souterrains are stone-lined underground passages or chambers associated with early medieval settlement in Ireland, generally thought to have served as cool storage spaces or places of refuge. The Ballybreen example appears to have been more elaborate than a simple linear corridor. Two depressions to the north, both showing visible stonework, suggest the original passage was L-shaped or T-shaped in plan, branching away from the main run rather than proceeding in a straight line. It is into these collapsed or partly open sections that badgers have dug their setts, and a further two setts some six metres to the north-north-east may also follow the line of the underground structure. The animals' instinct for dry, sheltered ground with workable soil has, in a roundabout way, traced the archaeology from below. A curving field boundary lies approximately twenty-one metres to the north, its arc perhaps echoing the outline of the cashel enclosure that once organised this whole stretch of ground.