My friend and I went to Kilkenny on a Sunday. From what we were told, the train did not run out there on Sundays - so we opted to take the bus.
It was about a 2 hour drive from Dublin and was a nice drive. I purchased my roundtrip ticket (which was 11.50 Euro) at the tourist center in Dublin. You can also purchase tickets at the bus station and before boarding the bus.
I got the most recent bus schedule while at the bus station in Dublin. The last bus leaving out of Kilkenny to Dublin was a little after 9pm (if I recall correctly). You have some flexibility with many different departure times.
The bus station in Kilkenny has a small area that has snacks, sweets and drinks.
The bus trip from Dublin to Kilkenny was only a few hours and inexpensive. It was comfortable ride to Kilkenny, BUT the way back was overbooked. Some people were standing the entire way and it was a longer drive due to traffic. The seats fill on a first get on the bus basis, so even if you bought a ticket first you may not necessarily board first. None-the-less, I would recommend this mode of transportation to and fro Dublin.
You can get here by bus ,train, car.Ckeck out
www.buseireann.com and www.irishrail.com for current prices and timetables on both buses nad trains.
This town is small so you can walk around it (trust me it's so small).The streets are really narrow for cars but there is a bypass and if you are driving I'd advise you to take this route.Killarney doesn't have a lot of traffic but it is just very slow moving.
Kilkenny (Patrick street) – Clonmel (railway station depart 14.35 arrive 15.25
Then take the Cahir bus from Clonmel at 15.45, and arrive in Cahir (Castle Street) at 16.10 and at 16.40 get the bus to Cashel. You should arrive in Cashel at 16.55. I know you have to take a lot of buses but this is the quickest way, taking 2hrs 20mins.