A souterrain, for those unfamiliar with the term, is a stone-built underground structure associated with early medieval Irish settlement, typically used for storage, refuge, or both. Most visitors to Muckross walk straight past this one, and it is easy to see why: much of it has fallen in, the base is silted up, and what remains visible is a low, roofless trench of limestone rather than anything that announces itself.

The structure sits approximately 170 metres east of Dundag Point and consists of two separate passages set at right angles to each other. The longer of the two runs roughly northeast to southwest, measuring about 4.8 metres in length and only 0.25 metres wide in places, with corbelled limestone walls, meaning the stones are layered inward to form the sides without mortar. This passage broadens into a small oval area before being cut off by a modern path. At right angles to it, beginning about 0.5 metres from its northeastern end, a second passage runs roughly three metres and narrows at its southeastern end to a lintelled entrance, barely 0.3 metres wide and 0.7 metres high, leading into a small oval chamber. That chamber, roofed with flat lintel stones, measures around 1.5 metres by 1.4 metres and stands about a metre high, constructed in the same corbelled technique as the passages. The overall plan, with its tight angled turn and constricted entrance, is consistent with souterrains designed to make unauthorised entry difficult.