Inisfallen Island, Killarney
If you look across the lake with Ross castle to your back you can seen Inisfalen island. Take a boat trip from the Castle to the Island. There are ruins of a 6th century monastery there. It's usually quiet, at most 50 people on the island at any given time. There are about 5 buildings to be seen there. It's also a really nice place to relax in the sun.
Off the beaten path
Take a fishing boat from Ross castle and row to Inisfallen island to see the ancient ruins of a monastery & on to O'Sullivans cascade. The monastery was built in the 6th century. If you are feeling fit, row on to the next Island beyond Inisfallen and over to the lake shore under the mountain to O'Sullivans cascade. The waterfall can also be reached by land by going to Fossa, taking a left as you leave the village and a further left about 3 km down the road. You'll have to do some trekking to get there, about a 2 Km walk.