The structure in question is a souterrain, an underground passage or chamber of the kind commonly built during the early medieval period in Ireland, often used for storage or as a place of refuge. What makes this particular example quietly curious is that, despite carrying a name suggestive of something known and visited, it left no mark on the Ordnance Survey's six-inch map when surveyors recorded the area in 1838. The place was named, yet apparently not mapped; known, yet already on its way to being forgotten.

The souterrain sits within what may be a cashel, a type of early medieval stone enclosure, typically circular, built to define and defend a farmstead or settlement. The possible cashel at Tullira has been assigned a record in the national monuments register, though the relationship between the enclosure and the underground structure it contains has never been fully investigated. McCaffrey, writing in 1952, placed the souterrain in the south-western quadrant of the interior but noted that it was inaccessible at the time of recording. Whether that inaccessibility was a matter of collapse, infill, overgrowth, or something else is not stated. The site exists in the record in a state of suspension, named and noted but not examined.