SIM local shop in arrivals Terminal 1 area after exiting baggage reclaim. Stocks SIM packs and phones. If arriving into T2 walk through to T1 via bridge.
Usually open 0800-1900.
Personally I recommend using Three Ireland if you get one from them. Good network and fast data. You can top up by EUR20 and it gives you
All You Can Eat Data
Unlimited Any Network Texts
Unlimited Any Network Weekend Calls
Free Three to Three Calls
http://www.three.ie/
The guys in SIM Local will set you up and explain things to you I have had to get something in there before and found them ok but not for a while.
If you have a phone that will work on the Irish networks unlocked then you could bring it and just buy a SIM pack for around EUR10 and top up with EUR20.