At Cregganna Beg in County Galway, within the enclosing stone walls of a cashel, a structure that may once have existed underground has left almost no trace at all. A cashel is a type of early medieval stone ringfort, its circular wall designed to enclose a farmstead or settlement. Inside this one, the evidence for something beneath the surface amounts to little more than a hollow in the ground.

In 1912, a researcher named Holt recorded what he described as a depression in the garth, filled with rubble stones. The garth is simply the enclosed yard or interior space of such a fortified enclosure. That sunken, stone-choked patch was enough to suggest the possible presence of a souterrain, an underground passage or chamber built into the ground, typically from drystone walling and roofed with large slabs. Souterrains are found across Ireland in association with early medieval settlements, used variously for storage, refuge, or ventilation. Whether one ever existed at Cregganna Beg remains uncertain. No visible surface trace of it survives today, and what Holt observed has since been obscured entirely.