The Dublin bus 747 service is every 15 minutes and takes an hour or there abouts to get to Heuston. You can book on line but I would guess thats out. so you would have to tender the exact fare when you board the bus. Problem I have is that trawling through the Dublin Bus web site I cannot find how much this is... I have emailed them and explained that my engineering degree did not cover the cost of bus fares and hopefully I will be able to give you a price when they get back to me.

The fee is 6euro each so you may have to buy something from one of the newsagents in Arrivals, to get some small change