In July 2010, Spike Island in Cork Harbour was handed over to Cork County Council, and it is now in the process of being turned into an impressive tourist attraction.
You can see the official website here.
The trip to Spike involves a little boat ride leaving from near the Titanic Bar in Cobh. The boat will take you the long way around Haulbowline (the Navy base), and you can spend a few hours on Spike itself. Just check when you land what time the ferries back are!
A family ticket will set you back €20, and is worth it.