A souterrain is an artificially constructed underground passage or chamber, typically built during the early medieval period in Ireland, and used variously for storage, refuge, or as an adjunct to a nearby settlement. This one left no surface trace at all, making it, in practical terms, a place defined entirely by its absence.

Local information recorded in the National Museum of Ireland file describes the discovery of what people called a "cave" at this spot, which was subsequently closed up again. The circumstances of that closure are unrecorded. What is clear is that the souterrain sits within a cluster of early medieval features: a motte, the raised earthwork mound associated with Norman fortification, lies roughly 150 metres to the south-southwest, while the remains of Balreagh church and its graveyard sit approximately 160 metres to the south. That concentration of monument types on a relatively small patch of rising ground suggests the area was a focus of activity across several centuries, from the early medieval Gaelic period through to the Norman reorganisation of the Irish landscape after the twelfth century. Whether the souterrain predates the motte, or was contemporary with the ecclesiastical site nearby, is not something the available evidence settles.