The earthwork itself is a univallate rath, a single-banked enclosure of the kind that served as a defended farmstead in early medieval Ireland, and on the surface there is little to suggest the careful engineering buried beneath it. The circular stone hut that once stood inside the rath has long since dissolved into a scatter of loose stones, but the L-shaped souterrain that opened from within it remains largely intact, its drystone walls and flat lintelled roof slabs still doing the work they were built to do centuries ago.

A souterrain is an underground stone-built structure, typically associated with raths and used for storage, refuge, or both. The example at Duagh was first formally noted by the Co. Kerry Field Club in 1945 and later described in detail in J. Cuppage's 1986 archaeological survey of the Dingle Peninsula. The structure runs to approximately 9.2 metres in total, beginning with a drystone-lined hollow just under a metre deep that gives onto a low lintelled opening, barely half a metre high, requiring a person to crouch or crawl to enter. The first passage section runs north-west for 3.7 metres, roofed by eight flat slabs and narrowing from 0.79 metres at the entrance end to just 0.45 metres at the far end, before the passage bends and continues north-north-west for a further 5.5 metres. That second section widens slightly as it goes and reaches a maximum height of 1.4 metres. At its far end, where a wall was broken through in relatively recent times to improve access, a short creepway once connected the passage to the terminal chamber. The original passageway through that wall has been largely destroyed, though the porthole slab that closed its northern end remains in place, its aperture, now blocked, measuring just 36 centimetres wide by 38 centimetres high. The chamber itself is the most substantial part of the complex: 4.9 metres long, 1.3 metres wide, and 1.8 metres high, its side walls inclining slightly inward toward six roofing slabs that cap the space at a width of around one metre. The precision of construction throughout, given that it was all done without mortar, is quietly remarkable.