Built around 1560 by Daniel O'Donovan, this imposing rectangular fortress rises four storeys high, measuring 14.2 metres east to west and 8.2 metres north to south. Though its southwest corner had collapsed over the centuries, recent conservation work by the Office of Public Works has stabilised this National Monument, ensuring visitors can still appreciate its defensive architecture and the strategic position it once held at the mouth of an isolated mountain valley.

The castle's military design is evident throughout its structure. A pointed arch doorway on the western wall leads into what was once a fortified lobby, whilst a spiral staircase in the northwest corner winds up through all four floors, its upper section cleverly protected by overlapping stone lintels. The ground floor features a distinctive pointed vault running east to west, constructed using the traditional wicker-centring technique. Each floor reveals different defensive and domestic features: gun loops flank the window embrasures for protection, whilst fireplaces on multiple levels provided warmth to the O'Donovan family and their retainers. The third floor showcases more refined square-headed windows with mullion and transom divisions, suggesting this level served as the principal living quarters.

Archaeological excavations conducted between 2002 and 2008 have added to our understanding of life at Castle Donovan, whilst old photographs from the 1930s reveal lost features including machicolations above the doorway and battlements along the south wall. Today, bartizans still crown the northeast and southeast corners, testament to the tower house's dual role as both fortress and home. As one of Cork's heritage castles now in State care, Castle Donovan offers visitors a chance to explore a well-preserved example of the tower houses that once dotted the Irish landscape, symbols of both local power and the turbulent history of Gaelic and Anglo-Norman Ireland.