Yet somewhere beneath the ground at this celebrated monastic complex lies a souterrain, a man-made underground passage or chamber typically built during the early medieval period, that remains entirely invisible at surface level. No hollow in the turf, no capstone poking through the grass, nothing to suggest its presence to the casual visitor walking among the ruins.

The structure was identified during a survey of the Clonmacnoise area carried out in 1987, referenced in a report by McDonald. Souterrains are found across Ireland in association with early medieval settlements and monasteries, and their precise function has long been debated. Theories range from food storage, taking advantage of the consistently cool underground temperature, to places of refuge during periods of raid or conflict. At a site like Clonmacnoise, which suffered repeated Viking and later Irish attacks throughout the ninth and tenth centuries, either purpose would have made practical sense. The 1987 survey placed this one on record, though the nature of the evidence suggests it was detected rather than excavated.