A souterrain, to give it its proper name, is a type of underground stone-built passage or chamber associated with early Irish settlement, most likely used for storage, refuge, or both. This one came to light in 1998, not through any planned excavation, but through the practical business of laying pipes.

When contractors broke ground along the main street that year, they partially exposed a structure consisting of one chamber with at least two passages leading off it. The walls and passages were built in drystone masonry, meaning the stones were fitted together without mortar, and the whole thing was roofed with large, flat slabs. The find was noted in Archaeology Ireland in 1998, offering a brief but tantalising glimpse of what lay underfoot in this otherwise quiet Longford village. Ballinamuck is already a place with a particular place in Irish history, known for the 1798 battle in which French and Irish forces were defeated by Crown troops, and it turns out the ground there holds older stories still. The souterrain has since been infilled, covered over once the pipe-laying work was completed.