Ross Castle

Situated in Killarney National Park and overlooking Lough Leane it is the only castle in the park. 

It was built in the late 15th century by one of the O'Donoghue Ross chieftains. Castle is now under OPW care (Office of Public Works).You can cross the lake to the other side via one of the locals with a small boat. There are certain times of the year you cannot visit the castle. I would check out those times.