Built in the early 1600s by the MacCarthy Muclaghs, a minor sub-sept of the MacCarthy Riabhach clan, this two-storey structure with its attic level represents the kind of defensive dwelling favoured by Gaelic families during a period of increasing English influence in Munster. The main block stretches 17.6 metres northwest to southeast, with two protective arms extending from its northeastern wall, creating a defensive courtyard arrangement typical of the period.

The castle’s defensive features tell a story of uncertain times; machicolations once crowned the side walls of the projecting arms, supported by tapering masonry corbels with intervening lintels, though these are now poorly preserved. A gun loop pierces the southwestern wall beside an original fireplace complete with bread oven, reminding visitors that this was both a home and a fortress. Six gables originally crowned the structure, with five elaborate chimney stacks featuring oversailing caps still visible, though one on the southeastern side is now ivy-clad. The southwestern wall stands only to ground floor height, whilst the rest of the building remains remarkably intact to its full original height.

Following its abandonment as a defensive residence, Coolnalong Castle found new life as an agricultural building and dwelling associated with nearby Durrus Court. This transformation saw significant alterations; window and door openings were adapted for wooden frames, an elliptical archway was inserted in the northwestern wall to provide access to a lean-to structure built inside the house, and the northwestern arm was converted into a separate dwelling with a dividing wall cutting it off from the main block. Despite these changes, the castle remains an evocative monument to the MacCarthys’ former power in West Cork, protected since 1955 under a preservation order that recognises its importance to Ireland’s architectural heritage.