Built by the O’Sullivans of Formoyle and Ballycarna, a sub-sept of the O’Sullivan Mores, this tower house saw action during the Cromwellian conquest of Ireland. In 1641, Owen O’Sullivan was wounded during a skirmish with Cromwellian forces at nearby Ballinskelligs, marking the castle’s place in this pivotal period of Irish history.

Today, only the southern wall and fragments of the eastern and western walls remain of this once-formidable structure. The surviving south wall stretches for just over 14 metres and rises to approximately 6.4 metres at its southwestern corner, built from roughly coursed slabs and rubble bound with lime and fine gravel mortar. The walls, about 1.4 metres thick with a slight base batter, hint at the castle’s original defensive capabilities. Archaeological evidence suggests the ground floor housed a main chamber with a smaller vaulted mural chamber at the southeastern corner, likely accessed through an entrance lobby in the now-collapsed eastern wall. The first floor appears to have been supported by wooden beams resting on rough corbels, with one such support still visible in the southern wall.

Much of Fermoyle Castle’s stone has found new life in the surrounding landscape; locals report that materials from the castle were incorporated into Fermoyle House and neighbouring farm buildings. Indeed, several punch-dressed sandstone blocks, likely pilfered from the castle’s original construction, can be spotted in the old agricultural structures nearby. While the castle itself may be largely gone, its stones continue to form part of the local architectural fabric, a fitting legacy for a stronghold that once protected this corner of the Iveragh Peninsula.