It is a souterrain, a type of artificial underground structure built during the early medieval period in Ireland, typically consisting of one or more dry-stone chambers connected by low, narrow passages. They are found in their hundreds across the island, yet each one carries its own unanswered questions: who built it, what it was used for, and how much of it survives intact beneath the soil.

Souterrains were most commonly constructed between roughly the seventh and twelfth centuries, usually in association with a nearby settlement or ringfort. Their precise function has long been debated. Refrigeration of perishable food, storage, and refuge during raids have all been proposed, and it is likely that different examples served different purposes at different times. In Connacht, where the landscape retains a high density of early medieval activity, the presence of a souterrain in Tully fits a broader pattern of agricultural and defensive organisation that shaped this part of Sligo long before any written record of it survives.