The early fortifications around the large inlet were concentrated close to the City of Cork and the adjacent Cobh Anchorage. At this time, Kinsale which was further down the coast, was used as the major naval base by the British.However, during the Napoleonic War, Cork was made the center of naval operations and Cork Harbour became a key location for the blockade of France. A naval dockyard was built on Haulbowline Island which is still used by the Irish Naval Service. Fort Westmorland was built on Spike Island, and a Martello Tower on Haulbowline Island.
By 1861 work had started on building Fort Camden and Fort Carlisle which covered the entrance to Cork Harbour.
In the early part of the 20th Century Fort Templebreedy was built further out from the harbour and armed coastal defence guns.
In 1923 Cork Harbour was retained as a British Royal Navy Treaty Port and remained in their hands until withdraw on 11th July 1938.