But drop your eye below ground level and the structure becomes quietly remarkable: a narrow underground passage, stone-walled and once roofed with enormous flat slabs, built with a small side chamber tucked off to one side, the whole thing angled south-south-west to north-north-east within the cashel's interior. A souterrain, as these underground stone-lined passages are known, would have served a community as storage space, a place to keep dairy produce cool, or in times of danger, as a refuge. This one also has a second, smaller alcove at its southern end, barely twenty centimetres square, the purpose of which remains, like so much about these structures, a matter of reasonable speculation.

The antiquarian T. J. Westropp recorded the souterrain in 1905, measuring it at just over eighteen and a half feet long and five feet wide, roofed by large thin slabs, one of which ran to seven and a half feet by five feet four inches. He also noted the small side apartment near the southern end, only four feet by three feet, a space that would have been a tight squeeze for an adult. By 1980, when George Cunningham documented the monument, it had collapsed. The passage that can be seen today is shorter than Westropp's measurement suggests, running to about 4.8 metres, with a height of only 0.7 to 0.8 metres under the remaining lintels. At the south-western end, the side walls converge into a V-shape, three lintels still in place overhead, forming a low chamber that retains an almost deliberate, architectural quality despite the centuries of neglect and partial collapse.