A souterrain is an underground passage or chamber complex, typically associated with early medieval settlement in Ireland, often interpreted as a place of refuge, storage, or both. The Coomnakilla example sits close to a hut site as well as the burial ground, suggesting it was once part of a wider settlement cluster. Following the landowner's discovery, a preliminary inspection took place in October 2011, with a fuller survey carried out afterwards by Julianna O'Donoghue Archaeological Services. What they found was a complex of some intricacy, despite significant collapse and damage. The network runs through four sub-rectangular or sub-circular chambers connected by narrow creepways, the deliberately constricted low openings, sometimes barely half a metre wide, that force anyone moving through a souterrain to crawl on hands and knees. Passage 1, aligned east to west, retains only one of its original lintels. Chamber 1 at its eastern end, accessed through a creepway just 0.45 metres wide, preserves a domed roof. Chamber 2 at the western end is largely unroofed. Passage 2 has collapsed entirely, and Chamber 3 beyond it is almost completely destroyed. The sequence ends, however, on a more intact note: Passage 3, on a north-north-east to south-south-west axis, retains all seven of its roof lintels, and leads through another constricted creepway into a small sub-circular Chamber 4, its dome still standing. No trace of the original entrance to the whole system survives, which means whoever last used it, and whenever that was, left no obvious sign of how they came and went.