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 Leixlip and Dún Laoghaire?
You can take the train to Dún Laoghaire at Leixlip (Confey) or Leixlip (Louisa Bridge), which arrives at Dun Laoghaire (Mallin).
How long does it take from Leixlip to Dún Laoghaire and when do the trains leave?
On average, it takes 1 hour and 5 minutes to get to Dún Laoghaire by train, but you will need 1 hour with the fastest trains. The first one is available at 06:01, whereas the last one leaves Leixlip at 16:01.