A souterrain, which is an underground stone-lined passage typically built during the early medieval period in Ireland, runs eastward from that narrow entrance, its roof made of flat lintels laid across the walls above. The opening itself is modest, just sixty centimetres deep at the threshold, and easy to miss in a grassy bank. But follow the linear hollow it leads into and you are tracing a passage roughly seven metres long, threading beneath the interior of the enclosure.

The rath itself, a circular earthen enclosure of the kind built by farming families across Ireland between roughly the fifth and twelfth centuries, provides the context for the souterrain's existence. Such underground passages were commonly integrated into raths, serving as places of refuge, storage for perishables, or both. At Knockadoo, the souterrain sits in the south-western quadrant of the rath, and at the eastern end of the hollow, two stone slabs protrude through the sod, identified as probable roof lintels. They are the most visible sign that the passage beneath has not entirely collapsed, and that its structure, however buried, remains at least partially intact.