It is a souterrain, a type of man-made underground structure built during the early medieval period in Ireland, typically consisting of one or more stone-lined chambers connected by low, narrow passages. They are found across the country in considerable numbers, and their precise purpose is still debated; current thinking suggests they served as cool storage spaces, places of refuge, or both. The one at Knockmore sits quietly in that category of known-but-not-yet-described places, present on the map, awaiting fuller attention.