The structure is a souterrain, an underground passage or chamber complex constructed from carefully stacked dry stone, of the kind built across early medieval Ireland and typically associated with nearby settlement enclosures. This one stretches to more than 15.7 metres in total length, and what makes it particularly notable is how well it has survived, its walls and layout remaining coherent enough to measure and map in precise detail.

The souterrain sits within the north-eastern sector of a rath, a circular earthen enclosure of the sort that once served as a farmstead or defended homestead during the early medieval period, roughly the fifth to twelfth centuries. The relationship between the two is typical: souterrains are frequently found tucked within or just inside the banks of raths, suggesting they functioned as places of refuge, storage, or concealment connected to whoever lived within the enclosure. At Marblehill, the underground complex is arranged across three chambers linked by two creeps, the narrow, low connecting passages that force anyone passing through to crouch or crawl, an architectural feature thought to slow or deter intruders. The first chamber runs east to west and measures 6.6 metres long; a creep off its north-eastern end leads into a second chamber oriented north to south, measuring 5.4 metres; and a further creep off that chamber's north-western end connects to a third, which again runs east to west. Entry into the complex today is through a broken roof lintel in that second, middle chamber. The third chamber is currently inaccessible.