The structure is a souterrain, an underground chamber or tunnel built from drystone construction and roofed with flat slabs, typically associated with early medieval settlement in Ireland. They were dug or built beneath or alongside raths, the circular earthwork enclosures that once served as farmsteads, and their precise function is still debated; theories range from food storage and refuge to simple ventilation shafts for subterranean spaces. At Cnoc na hAbha, the souterrain takes an L-shaped plan, running northwards for three and a half metres before turning east for a further two and a quarter metres. The passage is only about seventy centimetres wide and a little over a metre high at its tallest, a space that demands both crouching and a degree of nerve.

The souterrain sits within a small univallate rath, meaning an enclosure defined by a single earthen bank and ditch, positioned on the slope with views down towards the harbour below. Access to the underground passage is gained through a narrow opening in the outer face of the bank, positioned directly east of the rath's entrance, and the opening itself lies between two of the seven flat roofing slabs that cover the passage. One of those slabs, at the eastern end of the tunnel, has collapsed inwards over time, and the disturbance it caused may be visible on the ground surface as one of two slight depressions. The site was documented by J. Cuppage in the 1986 Dingle Peninsula archaeological survey, a detailed regional study of the Corca Dhuibhne area published under the title that reflects the peninsula's Irish-language heritage.