Donegal Castle stands on a rocky outcrop above the River Eske in the centre of Donegal town, a few steps from the Diamond. It is the former stronghold of the O'Donnell clan, who ruled this territory for centuries as one of the most powerful Gaelic dynasties in Ulster. The castle was burned by the O'Donnells themselves before the Flight of the Earls in 1607 to prevent it falling into English hands, then rebuilt by the planter Basil Brooke in the early 17th century.
The building that survives is a hybrid: the tall Gaelic tower from the 15th century stands next to a Jacobean manor house added by Brooke, with large mullioned windows and decorative stonework that looks out of place beside the older masonry. The interior has been restored with period furnishings and is open for guided tours. The castle is well-presented and the guides are informative on the history of both the O'Donnell clan and the Plantation period.