I agree that a car is a must unless you want to take a lot of bus tours, the rail system is almost non-existent except for the light (electric) rail system running to the south afor Dublin and a few suburbs/towns nearby.
If you do the Ring of Kerry in a car make sure to go clockwise because if you go anti-clock you will stuck behind a lot of tour buses which always seem to go in that direction and you will never get by them unless they stop at a viewing site.
Mike