The entrance, exposed by a collapse of the ground surface, is less than half a metre high, and the narrow creepway connecting the main passage to an inner chamber is tighter still. This is a souterrain, a type of underground passage and chamber built in drystone construction, meaning no mortar, just carefully placed stone, and found at early medieval settlement sites across Ireland. Their exact purposes are still debated, with storage, refuge, and ventilation all proposed, but the sheer effort involved in their construction suggests they were considered well worth building.

The Carrowneden souterrain sits within the south-western quadrant of a cashel, a roughly circular enclosure defined by a stone wall, which in this case has its own separate monument record. The underground structure itself is built from randomly coursed limestone blocks laid horizontally, with massive limestone lintels forming the roof. A steeply sloping passage leads down to the west-south-west before levelling out, and from the far end of that passage a low creepway, only about 1.7 metres long, curves slightly northward into a rectangular chamber measuring roughly 3.37 metres by 1.58 metres, with a ceiling height of around 1.46 metres. One detail stands out among the careful stonework: the west side of the creepway opening is flanked by a single large upright limestone slab described as being of unusual convoluted shape, its irregular form bridged to the main passage roof by two small pad stones. Near the north end of the creepway, just before it opens into the chamber, the floor is paved with a single slab that sits proud of the surrounding surface, a small but deliberate feature whose purpose is not entirely clear. The chamber itself has a rough lintelled recess cut into its north-north-west corner just below roof level, though a heap of displaced earth and stone from that recess now fills much of the chamber's northern end.