A promontory fort uses the natural advantage of a headland, cutting off the landward approach with walls and ditches rather than enclosing the full perimeter, and Doonamo did this with considerable complexity. The defended area stretches approximately 140 metres long by 85 metres wide, and the main stone wall, built in large drystone random-coursed masonry with a rubble core, originally stood around 2.7 metres high according to nineteenth-century observers, though it now reaches only 1.8 metres at its tallest point. Beyond it lies a steep-sided, stone-revetted ditch seven metres wide and nearly three metres deep, then a second stone wall, then the traces of a further shallow ditch with a counterscarp bank. What survives of the entrance is itself a puzzle of flagstones, grassed-over depressions, and the footprint of several small huts that once guarded the causeway.

For much of the nineteenth and early twentieth centuries, visitors and scholars including Richard Pococke in 1752, John O'Donovan in 1838, and T. J. Westropp writing in 1912 were struck by a field of upright limestone slabs outside the defences and interpreted them as a chevaux-de-frise, the term for a barrier of closely set pointed stones intended to slow an infantry advance, as seen at the famous fort of Dún Aonghasa on Inis Mór. The 43 surviving uprights at Doonamo average only about 25 centimetres high and show no regular pattern. Excavation of the feature in 1992 by Markus Casey suggested a different explanation entirely: that these stones are the remains of a children's burial ground, not a defensive obstacle. The two interpretations are not easily reconciled by looking, which is part of what makes the site quietly unsettling. Equally troubling is the evidence that some of the fort's more elaborate entrance features, including mural guard chambers described by Westropp as flanking the gateway, were dismantled not by time but by the local FCA, the Irish army reserve, who appear to have used the site as a firing range and reworked the earthworks to suit their purposes at some point in the twentieth century. The interior holds further compressed layers of occupation: the remnants of a cashel (a circular stone enclosure) reduced to its eastern and northern quadrants, traces of several hut sites, and two lengths of low walling that may represent a rectangular house.