The overgrown opening leads into a stone-lined passage, its ceiling formed from flat lintels laid horizontally across the walls, a construction method that requires no mortar and no arch, just careful selection of heavy stone. Beyond the visible section, the passage silts up and disappears, and a shallow depression roughly three metres long continues westward from the mouth, tracing the buried line of the structure beneath the turf.

Souterrains, from the French for underground passage, are subterranean stone-built chambers associated primarily with early medieval settlement in Ireland, broadly dating from around the seventh to the twelfth century. They are found across the country in their hundreds, often connected to ringforts or other enclosed farmsteads, and their exact purpose has long been debated. Refuge, food storage, and concealment have all been proposed, and the answer may differ from site to site. The one at Tonn Láin follows a familiar pattern: a narrow, carefully constructed passage, built to last, now largely reclaimed by the earth that once surrounded it. The lintel-roofed section that remains visible is enough to make the technique legible, the stones fitted with a plainness that is more impressive for its lack of ornament.