How nice that you'll be with us next autumn!
According to the Rome 2 Rio website
, you have multiple train options to get to Dublin Heuston (you'll still need a short shuttle bus ride to the airport after that). I would recommend switching in Limerick (Colbert) instead of Athenry, as the transfer times seem more comfortable. Now of course it depends on the time of the day and both options are quite similar. You can get tickets ahead of times through the Irish Rail website
I'm eager to hear what you've got planned during your tour!