That something, according to those who have long known the field, is a souterrain, an underground stone-lined passage or chamber built during the early medieval period, typically used for storage, refuge, or both, and often associated with exactly this kind of enclosed settlement.

The rath itself, a circular earthwork enclosure of the kind that dots the Irish countryside in the thousands, forms the broader context here. Such enclosures were the farmsteads of early medieval Ireland, their banks and ditches marking out a defended domestic space. Souterrains were frequently constructed within them, cut into the ground and corbelled or lintelled over with stone, then buried. At Tullylin, the raised area in the south-west quadrant of the rath has not been excavated, and so the tradition that a souterrain lies beneath it remains unconfirmed, the ground keeping its own counsel. What is notable is that the knowledge of it has persisted locally at all, passed on without the intervention of any formal investigation, the landscape remembered through use and conversation rather than through the spade.