If you are flexible with your travel dates, always consider that you can buy extremely cheap train tickets if you avoid travelling in the weekends and if you choose to travel during the low season.
When travelling on such short distances, you can take into account also busses and carpooling options: with some luck you will find very cheap options, even if you book on short notice.
What are the departure and arrival stations in Killarney and Dún Laoghaire?
The trains to Dún Laoghaire can leave from Killarney and arrive at Dun Laoghaire (Mallin).
How long will it take you from Killarney to Dún Laoghaire by train and when do the trains leave?
With the fastest connections you will be in Dún Laoghaire in 4 hours and 14 minutes, but on average it takes 4 hours and 31 minutes. The first train headed to Dún Laoghaire is at 06:04, while the last one leaves Killarney at 17:04.