The fortress began its life in 1505 under Hugh Roe O’Donnell, head of one of Ulster’s most powerful Gaelic families. By 1566, the English Lord Deputy Sir Henry Sidney was so impressed he called it “one of the greatest that ever I saw in Ireland in any Irishman’s lands”, though the O’Donnells would soon burn and partially demolish their own stronghold in the 1590s rather than let English forces garrison it. Their resistance ultimately failed; the castle fell to the Crown in 1601, marking the end of Gaelic rule in Donegal.

The castle’s second act came with Sir Basil Brooke, who received it as a grant in 1611 and transformed the medieval tower house into something altogether different: a Jacobean manor house. Where the O’Donnells had built for defence, with thick walls and narrow windows, Brooke added elegant gabled wings with large mullioned windows at ground level, effectively announcing that the age of siege warfare had passed. His coat of arms still adorns the fireplace overmantel, and archaeological excavations have uncovered 17th century pottery and clay pipes from this period of relative peace and prosperity. Brooke even appears to have recycled architectural elements from the nearby Franciscan Friary, incorporating their decorated doorways and window heads into his new home.

Today, visitors can explore both phases of the castle’s history. The original O’Donnell tower house, though much altered, still dominates the complex, whilst Brooke’s manor house survives as an atmospheric shell with its distinctive T-shaped plan and five gabled bays. The site also preserves sections of the defensive bawn wall with its splayed loops, and a two-storey gatehouse complete with a corner bartizan. Recent archaeological work has revealed that the castle was built over an early medieval burial ground, adding another layer to this site’s long history. Now protected as a National Monument, Donegal Castle stands as a physical document of Ireland’s turbulent transformation from the medieval to the early modern world.