A souterrain, roughly 8.2 metres long and tall enough to stand in at 1.4 metres, runs east to west beneath the ground, its dry-stone walls capped by six large lintels. At its eastern end, the dressed stonework gives way to a low cave cut into the bedrock itself, extending a further 9 metres east to west before turning and running 5 metres to the north. Whether that cave was shaped by human hands or simply adopted as a ready-made extension of the passage remains an open question.

Souterrains are underground stone-lined passages, usually associated with early medieval settlements in Ireland, and typically interpreted as places of refuge, storage, or concealment. This one sits in isolation about 150 metres east of a nearby enclosure, which suggests it belonged to a settlement that has largely vanished from the landscape. What makes Killour unusual is the junction between the built and the natural, a deliberate merging of craft and geology that is not always found in sites of this type. The entrance was set above field level, a detail that implies the original ground surface outside was lower, or that visibility of the opening was deliberately maintained rather than hidden.