Roughly L-shaped, about twenty metres long and only a metre deep, it sits quietly in the western part of a rath, and it is almost certainly the ghost of something that once ran underground.

A souterrain is a man-made underground passage or chamber, typically stone-lined, built during the early medieval period in Ireland. They were constructed beneath or beside raths, the circular earthen enclosures that served as farmsteads, and are thought to have functioned as places of refuge, cool storage for foodstuffs, or both. The one at Knockaunbaun appears to have collapsed inward at some point, leaving behind a hollow rather than an intact tunnel. The shape of that hollow is itself informative: one leg runs north to south, while the other extends off the south-east end and turns east to west, giving the whole feature its angular outline. The overall length of around twenty metres would have made it a reasonably substantial structure in its original form, with a width of some two and a half metres suggesting a passage wide enough to move through with some ease. It sits within the bounds of a rath recorded separately in the area, meaning the souterrain was almost certainly a deliberate element of a wider early medieval settlement rather than an isolated feature.