A series of low mounds and shallow depressions in the south-east and north-west quadrants of the enclosure hints at something buried and broken beneath the surface, the probable remains of a collapsed souterrain.

A souterrain is an underground passage or chamber, typically constructed during the early medieval period in Ireland using stone-lined walls and large capstones laid across the top. They are found across the country, usually associated with ringforts or raths, the circular earthen enclosures that were the standard farmstead of early medieval rural life. Their precise function is still debated; they may have served as cool storage for dairy produce, as refuges during raids, or both. The rath at Bundoon is itself a recorded monument, and the souterrain, if that is indeed what the subsurface disturbances represent, would have sat within its interior, tucked into at least two separate sections of the enclosure. The qualification matters: what survives above ground is ambiguous enough that the identification remains cautious rather than confirmed, the landscape offering suggestion rather than proof.