This was our first parking experience in Ireland, we really had no idea where to park in Kilkenny and when we found a metered spot near the castle, we took it. The parking near the castle is via pay and display paybox but unlike the paybox system we have here in Chicago, I believe it only took currency and not credit or debit cards. We plunked in the €2 that we had with us and then went off in search of more change, after that I accumulated as much change as I could to use for future meters.
After paying at the box, you get a receipt which you display on your dashboard.
There is plenty of parking in Kilkenny right downtown. There is multi-story parking between Friary and Ormonde streets and another multi-story farther north off James's Street but our favorite place was the large lot by the river Nore and just behind the medieval section of Kilkenny.
If you approach Kilkenny from the south (heading north), you will eventually arrive at Patrick Street that comes to a Y. Take the right branch onto Rose Inn Street and just before the river, turn left. The parking lot is just along that road to the left and if you are there after six PM, you don't have to pay because the gates are opened for the night.
You can easily walk from this lot to anyplace touristic in Kilkenny and if you want to visit a pub in the evening, it's right behind two excellent ones, Kyteler's Inn and Bollard's Pub. Both actually have doors onto the parking area although we preferred the street entrances above.
If you look at the parking map on the web site below, the parking by the river is named: Dunnes Stores - Market Yard and it is number 2 on the list.
2 Hour drive from Dublin on the N9. Yes , you do have to drive through Carlow!
However there is good bus and rail connections from Kilkenny to Waterford, Dublin, Wexford, Cork and even Limerick