Hours of admission are different depending on what time you visit, so make sure to check the website for opening hours.
If you travel to Kilkenny Castle by car (like we did), take note that there is not a designated parking lot and street parking is very limited. You can park in the car park at the Market-Cross Shopping Centre on Parliament Street for €1.50 per hour.
End of April to early May is a good time to go, as the heavy tourist season hasn't begun. Plus, opening hours are longer than in the winter.