This low-lying green island in Cork Harbour was once an important part of the port's defences, topped by an 18th-century artillery fort. In the second half of the 19th century, during the Irish War of Independence, and from 1984 to 2004 it served as a prison, gaining the nickname 'Ireland's Alcatraz'. Today you can enjoy a guided walking tour of the former prison buildings, then go off and explore on your own; the ferry departs from Kennedy Pier, Cobh.
The guided tour takes in the modern prison, the old punishment block, the shell store (once used as a children's prison) and No 2 bastion with its massive 6in gun. Other highlights include the Gun Park, with a good display of mostly 20th-century artillery; the Mitchell Hall, with an exhibit on the Aud, a WWI German gun-running ship that was sunk in the entrance to Cork Harbour; and the Glacis Walk, a 1.5km trail that leads around the walls of the fortress, with great views of Cobh town and the harbour entrance. You'll need around four hours to make the most of a visit.