A souterrain, an artificially constructed underground passage typically associated with early medieval settlement and used for storage or refuge, had survived undetected just sixty metres south-east of St. Feichin's church, until the machinery arrived.

The structure was considerably damaged before anyone with an interest in antiquities could properly examine it, which means the account that survives is partial at best. What was recorded shows a passage roughly six metres long, 1.3 metres wide, and 1.25 metres high, running north to south with its entrance at the northern end. Towards the south it curved eastwards, though collapse prevented investigators from determining its full extent. The construction was modest: poor quality drystone walling, slightly corbelled at the sides, meaning the stones were angled inward to narrow the space before flat transverse lintels, each around 0.45 metres wide and 0.3 metres thick, were laid across the top to form the roof. It was functional rather than fine work, the kind of structure built to serve a practical purpose rather than to impress. The site sits in a valley already dense with early Christian remains, including the monastic foundation associated with St. Feichin, who established a community at Fore in the seventh century, making the discovery geographically unsurprising even if its survival was not.