That hollow marks the likely position of a souterrain, an underground passage or chamber built during the early medieval period, typically constructed from stone and used for storage, refuge, or both. The entrance has long since collapsed and been filled in, leaving just this quiet indent in the earth as evidence of what lies below.

Souterrains are most often found in association with ringforts, the circular enclosed settlements that were the dominant form of rural habitation in Ireland from roughly the sixth to the twelfth centuries. At An Drom Réidh, the souterrain sits within the ringfort recorded separately in the county's archaeological inventory. The filled depression suggests the passage roof gave way at some point, as often happens when the capstones spanning an unlined or drystone-built tunnel can no longer bear the weight of accumulated soil above them. What remains is, in essence, the scar left by that subsidence.