It is about eleven metres long and barely a metre wide, built without mortar in the drystone method, and it runs north to south. Two blocked-up breaches, one at each end, are the only points at which it can be examined at all, and even then only by peering in from above.

The structure is a souterrain, a type of underground passage or chamber built in early medieval Ireland, typically associated with settlement sites such as ringforts and cashels. A cashel is a stone-walled enclosure, the drystone equivalent of the earthen ringfort, and they are scattered across the west of Ireland in considerable numbers. Souterrains within them were likely used for storage, refuge, or both, taking advantage of the stable cool temperature underground. The Farranablake example sits within a cashel that has its own separate record, and the relationship between the two structures suggests this was once a functioning settlement of some complexity, even if very little of that life is now legible on the surface.