While the Leap card did seem like a good deal, be aware there is a 5 euro deposit and the only way you can get that back (along with any credit you have left on the card) is to mail it in and provide an EU bank account to have the money transferred over!... so basically I would only recommend it if your using public transportation A LOT or will be staying in Dublin for a while.
There was a deal for 72 hours of unlimited Dublin public transportation for 19.50 euro + leap card deposit if you are there for a few day stopover and plan to take the bus, link trains or downtown tram a lot. That is valid on the express airport buses too so that amount would be quickly reached.
I took the airlink express bus number 747 back to the airport for an early morning flight and it was worth the extra money if you're in a hurry. The route takes the tunnel out of downtown Dublin (which has some steep tolls so there isn't much traffic) getting me to the airport in around 15 minutes! That was probably just as fast as a taxi, but much much cheaper. The bus usually starts running at 5 or 6am depending on the day.