Here are the transport options I've thought of so far:
Drive there: 4 hr 15 min (incl a 30 min break halfway through for food.) Would need to leave about 05:45 if attending the 10:00 ceremony. This is most convenient option if you need to get back earlier or can't stay overnight. But 7.5 hours is a lot of driving for one person on one day (even professional drivers not recommended to drive more than 9 hours for one day). Might not be too bad if you can alternate the driving with your guest. If I left straight after the ceremony, I would arrive home about 16:00.
Train from Dublin Heuston: A train departs for Killarney about every two hours. Some are direct and some have one change. Tickets are about €46 round trip and the journey ranges from 3 hr to 3 hr 45 min travel time. The invitation letter states that there's a shuttle service from the station to INEC (the ceremony location). Going by train is very handy if you're not living too far out. For me, I'd have to travel an hour to reach Dublin Heuston and stay overnight in Killarney, as no trains arrive before 12:10. I would arrive home about 18:00.
Plane to Kerry Airport: I found two flights there and back every day, leaving Dublin at 12:40 and 18:45, and leaving Kerry at 07:30 or 14:10. Flights are about €80 - €100 round trip and are about 1 hour. The airport is about 20 min drive from INEC. Depending on your ceremony time: you may need to stay overnight before the ceremony, and advantage of this is it gets you back to Dublin Airport by 15:10, so you may be home even faster than if you were driving. I'd be home about 16:30.
What transport options are you considering?
There's pros and cons with all the transport, but I am just happy to have received my invitation!
