Dublin to Kilkenny isn’t far by train, only about an hour and a half, but by bus it takes an hour longer. It seems there’s not much difference in the travel time from Kilkenny to Tralee, but it’s still a long trek at least five and a half hours long. Personally, I prefer the train because the leg room is better (I’m quite tall), and I found the buses quite cramped. On the train you can get up and walk around, too. I had one experience where a bus was over an hour late, and barely had room for everyone waiting, but not with the train. You can purchase a bus/train pass so you can make use of both if needed, but it will depend how many times you are going to use it as to whether or not it’s economical for you.