A souterrain, to borrow the term from Old French, is an artificially constructed underground chamber or tunnel, typically stone-lined, built during the early medieval period in Ireland for storage, refuge, or both. What makes Kilberrihert quietly interesting is not just the presence of one such structure but two, along with the ghost of a third suggested only by a depression in the soil.

The ringfort itself, a roughly circular enclosure of the kind that once numbered in the tens of thousands across Ireland, forms the wider context here. Within or beneath it sit two recorded souterrains. A third was observed in 1934 by a researcher named Bowman, who noted a depression on the north-eastern side of the fort covering around twenty-eight square yards, interpreting it as evidence of a further souterrain that had subsided. No visible surface trace of that third passage remains today. The detail is a small but telling one: an observation made in passing nearly a century ago, preserved in print, pointing to something that has since quietly vanished into the ground.