These subterranean passages, found throughout Ireland but particularly common in the northern counties, were typically built between the 6th and 12th centuries. The Monreagh souterrain, also known as Barr of Kilmackilvenny, represents a remarkable example of these enigmatic structures that once served the local community, though their exact purpose continues to spark debate amongst archaeologists.

Souterrains were ingeniously constructed using stone slabs to create underground chambers and passages, often incorporating clever features like creeps; low, narrow sections that forced anyone entering to crawl on hands and knees. While some historians believe they were primarily used for storing food and valuables, others suggest they served as temporary refuges during raids. The cool, stable temperatures underground would have been perfect for preserving dairy products, grain, and other perishables, making them essentially the medieval equivalent of a refrigerated larder.