Souterrains, as these stone-lined tunnels are known, were built during the early medieval period, typically as storage spaces or refuges connected to the enclosed settlements above them. What makes this particular site quietly puzzling is that the evidence for them is now largely circumstantial: a series of hollows in the earthwork, each one a faint argument for something that was once deliberate and structural.

In 1934, a researcher named Bowman recorded two holes sitting in the northern and southern portions of the ringfort's rampart, each measuring twelve feet across at the top and six feet deep, and identified them as souterrains. A ringfort, to be clear, is a roughly circular enclosure defined by one or more earthen banks, the most common type of early medieval settlement in Ireland. At Corbally, at least one of the features Bowman described may correspond to a hollow that can still be identified at the junction between the interior of the fort and its northern bank. Two further hollows have also been noted, cut into the outer face of the bank to the south and north-north-east. These, however, are thought more likely to be the remains of lime kilns, structures used to burn limestone and produce quicklime for agricultural or building purposes, rather than any continuation of the souterrain system. The distinction matters: it means that what looks, at a glance, like a coherent underground complex may in fact be a mixture of early medieval tunnelling and later agricultural activity, superimposed on the same earthwork over a considerable stretch of time.