This is a souterrain, a type of stone-lined underground chamber or tunnel built in early medieval Ireland, typically for storage, refuge, or ventilation of adjacent settlements. What makes the Tullira example quietly peculiar is not just its reported shape, which is unusual even among souterrains, but the fact that it has left no mark whatsoever on the surface above it. The ground gives nothing away.

The chamber lies just outside the southern sector of a cashel, a circular stone-walled enclosure of the kind built by farming communities in early Christian Ireland, often to protect a dwelling and its livestock. The cashel at Tullira is a recorded monument in its own right, and the souterrain appears to have been associated with it, positioned at its edge rather than within the enclosure proper. The figure-nine shape, if accurate, would suggest a main passage curving back on itself with a terminal chamber, though the detail comes from local tradition rather than excavation. No archaeological investigation has confirmed the form, the extent, or the construction of what lies below, and no trace of it is visible from the surface today.