The collapse exposed an earth-cut souterrain, a type of underground passage or chamber excavated by early medieval communities in Ireland, typically for storage, refuge, or ventilation of nearby dwellings. This one had been cut into the local shale, and its roof sat just below the surface of the road, meaning that traffic had been passing directly over it, unknowingly, for who knows how long. For safety reasons, the souterrain was subsequently filled in with fine gravel, so the road above it is now functional again, and the passage below is sealed.

What makes the situation at Gortacurrig particularly intriguing is the possibility that this is not an isolated find. Back in 1993, a probable souterrain was recorded roughly thirty metres to the east, just south of the same road. Whether that earlier feature and the one exposed in 2005 are two separate structures, or two parts of a single more extensive underground system, has not been established. Souterrains of this kind were often components of larger early medieval settlement complexes, and where one appears, others are not unusual. The shale geology here would have made excavation demanding work, which lends a certain quiet weight to whatever community went to the trouble of creating these passages in the first place.