This is a souterrain, a type of man-made underground structure common in early medieval Ireland, typically built of dry-stone walling and used for storage, refuge, or both. The one at Caherbullog is a relatively well-preserved example, tucked inside the northern perimeter of the cashel, which is itself a stone-walled ringfort of the kind that once served as a defended farmstead across the Irish countryside.

The structure begins as a stone-lined hollow, roughly 3.4 metres long and varying in depth from 0.8 metres at the north end to 1.5 metres at the south, with an average width of just 0.8 metres. This leads into the main chamber, aligned north to south, which opens up considerably: 5.8 metres long, 1.9 metres wide at its centre, and 1.9 metres high, with twelve lintels forming the roof overhead. Near the southern end of the east wall there is a small recess, the purpose of which is unclear but which may have served as a niche for storage or a lamp. At the southern end of this chamber, a low opening measuring 0.95 metres wide and 1.15 metres high connects to a second, smaller chamber extending to the southeast. This inner room is 4 metres long, 1.25 metres wide, and only 0.75 metres high, roofed by six lintels, and accessible from the cashel interior through a separate small opening at its southeastern end. The result is a structure with two entry points and a change of direction mid-way, a layout that would have made uninvited access considerably more difficult.