A souterrain is an artificial underground passage or chamber, typically constructed from stone, and associated in Ireland with early medieval settlement. They served various purposes, most likely as cool storage spaces or places of refuge, and they turn up across the country in their hundreds. This one, however, leaves almost no trace above the surface.

The structure came to light during a systematic survey of the Clonmacnoise area carried out by McDonald in 1987. Beyond that identification, the record is spare: the souterrain is not visible at ground level, which makes it something of an archaeological ghost, known to exist but offering nothing to the casual eye. Clonmacnoise itself was founded in the sixth century and grew into one of the great centres of early Christian learning and craftsmanship in Ireland, so the presence of an underground feature of this kind is hardly surprising. Such passages are frequently found in association with monastic or domestic settlement, tucked beneath farmsteads and enclosures that have long since vanished from the landscape.