Local services 41 and 41B take about 20 minutes longer than the express bus but they're much cheaper (�1.90).
If you do decide to take the dedicated airport service, and intend to use the Dart rail system as well, you can buy a combined ticket for just one euro extra (i.e. �7).
Just doing a day trip thanks to the cheap Ryanair tickets on sale this weekend. I dont think they do day return discounts on the dedicated airport bus service!
Get one of the blue coaches as they run frequently and costs E7. Get off in O'Connell Street and everything is walkable from there. If you want to get around and get info on Dublin as well, the Hop on Hop off buses are good value as you can use them all day.
Sorry about late reply. 7 Euros single. I dont know if they do returns or day returns. We only used them from airport as we got a taxi back (28 Euros)!! But late in evening and we didnt want to chance being late to airport.