The castle consists of a rectangular main tower measuring 8.8 metres east to west and 8.15 metres north to south, with a smaller square tower tucked into its southwest corner. Built by Donogh More O’Mahony, whose carved likeness once adorned the north wall until it was stolen in 1972, the fortress served as a stronghold for the O’Mahony clan for centuries.

The castle’s defensive architecture reveals sophisticated medieval design. The main tower rises four storeys whilst the corner tower extends to five, with access between levels provided by an intricate system of mural stairs and passages. From the first floor, straight stairs built into the eastern wall lead to the third floor, whilst a spiral staircase in the southeast corner climbs to the wallwalk. The corner tower houses a garderobe chamber accessible from the first floor, and somewhat grimly, a doorway in the south wall opens onto an unlit dungeon below. The windows show a clear hierarchy; lower levels feature simple square-headed lights with splayed embrasures, whilst the upper floors boast more elegant twin ogee-headed openings.

The castle’s turbulent history mirrors the contested nature of this coastal region during the Tudor conquest of Ireland. Crown forces captured Dunmanus in 1602, only for the O’Mahonys to reclaim their ancestral seat soon after. Though the battlements have long since fallen and time has taken its toll on the structure, the remaining stonework still commands respect. A vault over the second floor, running east to west, remains intact, and the various doorways connecting the two towers at different levels demonstrate the complex internal layout that once sheltered generations of the O’Mahony family.