The structure is a souterrain, a type of deliberately constructed underground chamber or tunnel found across early medieval Ireland, typically built without mortar and associated with ringforts and raths. This one is aligned east to west, measures more than 4.6 metres in length, reaches a maximum width of 1.2 metres, and stands up to 1.8 metres high at its tallest point. Dry-stone built and entered from its eastern end, it is the kind of place that rewards slow attention.

The souterrain sits within a rath, the term used for a roughly circular earthen enclosure that served as a defended farmstead during the early medieval period, broadly from around the fifth to the twelfth centuries. Thousands of raths survive across Ireland in varying states of preservation, and many contain souterrains, which are thought to have been used for cool storage, refuge, or both. At Moyleen, a depression that extends eastward from the entrance before turning south hints that the passage may originally have been L-shaped in plan, a configuration that would have made it more defensible and harder to navigate quickly in the dark. That extra turn, if it existed, is now only readable as a surface feature rather than an intact passage.