A souterrain, an artificial underground passage or chamber typically built during the early medieval period and associated with nearby settlement, is recorded here within the enclosure of a ringfort. No surface trace of it remains visible today.

Souterrains were commonly constructed from stone-lined or rock-cut passages, sometimes serving as places of refuge or for the cool storage of food, and they are frequently found in association with ringforts, the circular earthwork enclosures that were the dominant farmstead type in early medieval Ireland. The ringfort at Carrowcrin is itself a separate recorded monument, and the souterrain sits within its bounds. Beyond that association, the record is notably spare: no dimensions, no description of construction, no account of how or when the souterrain came to be documented. It is a feature known largely through tradition or earlier survey rather than through any excavation or exposed stonework.