That depression, easy to miss and easier still to dismiss as natural subsidence, is the sole surface clue to what lies below: a souterrain, an artificial underground chamber of the kind built by early medieval Irish communities, typically as a place of refuge, storage, or cold keeping.

The souterrain sits within a rath, the Irish term for a ringfort, a type of enclosed farmstead defined by one or more earthen banks and ditches. Raths were the most common form of rural settlement in early medieval Ireland, and many were in use between roughly the sixth and twelfth centuries. The pairing of a souterrain with a rath is well attested across the country; the underground passages were frequently dug as a deliberate feature of such enclosures, sometimes leading from a dwelling into the surrounding bank. At Curragh, the entrance has been blocked at some point in the past, and what remains above ground is that single, quiet indentation at the centre of the rath. The detail comes from local information rather than excavation, which means the full extent, construction, and date of the chamber remain unknown.