The souterrain at Cahernabrock, an underground stone-lined tunnel of the kind built throughout early medieval Ireland, sits sealed and unexamined within the enclosure of a ringfort, its interior unknown to anyone who has tried to reach it in recent times. That inaccessibility is itself quietly interesting: here is a feature whose existence is recorded, whose location is known, and yet whose contents remain effectively a mystery.

Souterrains were constructed, typically between the seventh and twelfth centuries, as underground chambers or passages associated with ringforts, the circular enclosed farmsteads that once numbered in their tens of thousands across the Irish countryside. Their precise purposes are still debated, though storage of perishables and refuge during raids are the explanations most often advanced. The ringfort at Cahernabrock is itself defined by a low mound stretching some seventeen metres, enough to suggest a modest but deliberate enclosure. The souterrain was noted in a 1994 archaeological survey of the Ballinrobe district compiled by D. Lavelle, covering the area around Lough Mask and Lough Carra, and at that time it was already described as inaccessible.