Set within a ringfort, the circular earthen enclosure that was the standard form of rural settlement in early medieval Ireland, the depression at the centre of the interior has long been taken as a possible indicator of something underneath: a souterrain, the term for an artificially constructed underground passage or chamber, typically built from drystone walling or cut directly into rock, and used for storage, refuge, or both.

Souterrains are found across Ireland in considerable numbers, most commonly in association with ringforts, and their presence is often only betrayed by exactly this kind of subtle surface feature, a slight collapse or dip where the ground above a buried structure has settled over centuries. At Tullig, no excavation appears to have confirmed whether a souterrain actually lies below, leaving the depression as a suggestive clue rather than a certainty. The ringfort itself, recorded separately, provides the broader context: these enclosures were typically built and occupied between roughly the sixth and twelfth centuries, serving as farmsteads for individual family groups. A souterrain within such a site would have been an entirely ordinary feature of that world, though ordinary is perhaps not quite the right word for a hand-built underground chamber that has quietly outlasted everything built above it.