A souterrain, to use the archaeological term, is a man-made underground chamber or tunnel, typically built during the early medieval period in Ireland from dry-stone construction and used variously for storage, refuge, or both. The one at Goulacullin leaves no visible mark on the grass above it, and has not done so for the better part of a century.

Local tradition holds that the souterrain was discovered sometime in the 1920s and then deliberately backfilled, closing it off again. Why it was filled in is not recorded, though such decisions were not unusual; landowners sometimes sealed newly discovered underground features for safety reasons, or simply because they had no obvious use for an ancient subterranean tunnel opening up in a working field. The result is a site that exists more as memory and oral record than as anything a visitor could examine. The slope continues to be grazed, and the ground gives nothing away.