My parents are coming in August and they're staying here:
http://www.staydublin.com/The Millennium Tower apartments are just around the corner from the Luas line that will take you to Heuston station. It's self catering, so you'll save some money on meals and there are loads of shops on Henry Street where you can get food. You'll be within walking distance of pretty much everything in the centre.
If you were going to stay at this location, you could get the AirCoach to O'Connell Street, then you could walk to the hotel. Lilybelle is right that they are blue and they are right where you exit the airport. I'm not well versed on the buses here because I live in the centre so I walk everywhere or get the Luas, and, well, most of the buses you will see are out of service. Don't ask.
Have fun on your train trip! You will be WRECKED by the time you get back! And if you think about it, take some food on the train and don't rely on the buffet car. The prices are ridiculous and the food isn't great.