Thanks for getting back to us!
My colleague Rochelle and I have been looking at this and would recommend a website Rome2Rio to find the best options for you.Temple Bar to Heuston Station
is the first thing to look at with the Red line LUAS
or a walk depending on how you feel and what time you decide to travel. A taxi may be needed for an early trip.
There is a Train at 7 am towards Killarney that has a change in Mallow but it does take just over 3 hours still so arriving at 10:19 am. Irish Rail
website helps you to find this service and the return on Sunday. Online tickets for the dates mentioned are not available
, you would need to check with Irish Rail for more information as these may be only available at the ticket office or the service may not be running that day.
The bus takes an hour longer, again the Rome2Rio
website has details of the service I mentioned before. The distance is 309Km through some of the rural parts of the country so times of travel may seem long for the distance but a lovely view all the way!
Hopefully, this will helps with your plans and that you can find a way to get to Killarney even if it is just for one night.