Having trouble with my itinerary still. I'm curious, coming from either Kinsale or Kilkenny - would it be possible to see the Rock of Cashel AND Bunratty Castle in one day, staying in Shannon that night?
Yes, easily done. The distances aren't great at all. Cashel to Bunratty is only an hour's drive. Kilkenny to Cashel is about the same.
And Kinsale to Rock of Cashel is about 1:45, though it might be longer because it's not as direct a route as Kilkenny to Cashel. I would allow about two hours at Rock of Cashel, so the whole drive should be easily doable no matter which city you leave from.
Lizzy, you could easily stay in Bunratty that night instead of Shannon - if you are looking to fly out of Shannon. Bunratty was a cute little town and we booked a simple B&B (Bunratty Castle Mews) - explored the Castle, the village around it and had dinner at the original Durty Nellys. We were then up at 5am to drive back to the airport which was less then 20 minutes away.