That's great that you were able to change the flight - not having to make the transfer from the airport to the train station in Dublin will make your side trip so much easier.
On the way back, you can get the Luas tram from outside Heuston Station into the centre of Dublin (not sure where you're staying in Dublin) the trams run every few minutes and the journey will only be 5-10 mins.
Well done on making this work for your Father in Law! :)

Thanks Bumbly Wumbly! You have also given such great advice and enthusiasm. Haven't booked a hotel yet in Dublin. Any advice on best areas to stay for two day, that is the center of all of the sites, historic pubs etc? Again, I appreciate all the help:)