That combination, a roofed stone corridor built into the perimeter of an earthwork enclosure, is what defines a souterrain: an artificially constructed underground chamber or passage, typically associated with Early Christian period settlement in Ireland, and used variously for storage, refuge, or both.

The ringfort, or rath, into which this structure is built occupies the northern edge of a plateau, a position that would have given its inhabitants a reasonable outlook over the surrounding landscape. The passage itself is modest in scale, roughly 2.8 metres long, 1.4 metres wide, and just over a metre high, the kind of tight, crouched space that was never intended for comfort. A second souterrain lies nearby, closer to the centre of the same rath, suggesting that whoever occupied this enclosure invested some effort in providing underground infrastructure, whether for keeping food cool and dry or for withdrawing from view in less settled times. The two structures together make Tullassa a quietly notable example of how layered and deliberate early Irish settlement could be, even within a single modest ringfort.