It is a souterrain, a type of structure built during the early medieval period in Ireland, typically by hand-laying drystone walls and roofing slabs to create a tunnel or chamber beneath the ground. Their purposes are still debated: storage of perishables in the cool underground air, refuge in times of raid, or some combination of both. What is certain is that they were deliberate, skilled constructions, usually associated with a settlement or ringfort above ground. The one at Mullaghmarky is recorded as a monument, but beyond that, the documentary trail runs thin.