Three separate areas of collapse, running in a rough line from north-north-west to south-south-east for roughly fourteen metres, trace the buried course of a souterrain, one of those narrow underground passages constructed during the early medieval period, typically cut into the earth and lined with stone, that served as storage spaces, places of refuge, or both. Where the surface has fallen in at the south-eastern end, a cross-section of the structure becomes briefly visible: stone-lined walls, flat lintel stones laid across the top to form a roof, and a passage now silted up with centuries of accumulated debris.

The souterrain sits within a ringfort, a type of enclosed farmstead common across Ireland from roughly the sixth to the twelfth century, usually defined by one or more earthen banks and ditches surrounding a domestic interior. The association between ringforts and souterrains is well established; the underground passages were almost certainly built as part of the same settlement, providing cool storage for perishables or a concealed retreat in times of danger. At Mullenroe, the collapsed surface has done some of the archaeological work for observers, exposing the lintelled roof construction that would otherwise remain entirely hidden. The passage itself is silted rather than fully intact, meaning the interior is not accessible, but the structure and its orientation are legible from the surface disturbance above it.