Lonely Planet review for Ross Castle
Restored by Dúchas, Ross Castle dates back to the 15th century, when it was a residence of the O'Donoghues. It was the last place in Munster to succumb to Cromwell's forces, thanks partly to its cunning spiral staircase, every step of which is a different height in order to break an attacker's stride.
The castle is a lovely 3km walk from the St Mary's Cathedral pedestrian park entrance; you may well see deer. If you're driving from Killarney, turn right opposite the petrol station at the start of Muckross Rd. Access is by guided tour only.