A souterrain is a man-made underground passage or chamber, typically dry-walled and roofed with stone lintels, built during the early medieval period and associated with ringforts across Ireland. They were used variously for storage, refuge, and possibly as cool larders. The one at Tullig is notable chiefly for having been deliberately closed off, its entrance sealed by a former landowner, and for the fact that no one now knows exactly where it lies.

The only record of the structure comes from Broker, writing in 1937, who noted an underground chamber that had been closed in by a man named Nash, described as a late owner of the land. That single observation is all that survives. Nash's reasons for blocking the entrance are unrecorded; perhaps it was a safety concern, or simple indifference to what lay beneath his fields. The souterrain sits within a ringfort, one of the thousands of roughly circular earthwork enclosures built across Ireland from the early medieval period onward, typically as farmsteads enclosed by an earthen bank and ditch. The combination of ringfort and souterrain is common enough in the Cork landscape, but this particular example occupies an unusual position: it is known to have existed, its general setting is identified, and yet its precise location has been lost.