I agree that Kilkenny is probably a bit too far considering you have a noon flight. It's about a two hour journey, and you have to keep fingers crossed that you don't sleep through your alarm, have car issues, get caught in traffic, or have a hard time turning in your car at the lot. Here is what I have done many times, and it works great.

Arrive the afternoon before you fly home, turn your car in a day early. and take a free shuttle to one of the airport hotels close by such as the Clayton or the Premier Inn. I have stayed at both and they are quite nice for a good price. Get checked in mid afternoon and either relax at the hotel, or take a short taxi to Swords to have a nice dinner that evening ... some great pubs there and also many B&Bs if you want to stay there instead of a hotel. You can also get a train close by that will take you straight into Dublin and back for a small fee, and do the city exploration that evening.

The next morning, take the free shuttle to the airport, get dropped off right at your gate, and enjoy the rest of the morning. You might even get to sleep in a bit! This is a low stress way to end your trip, compared to doing several things that morning.

Have a great trip!

Geno