The DART commuter rail connects Dún Laoghaire to central Dublin (Pearse, Tara Street, Connolly) in about 20 minutes, with trains every 10–15 minutes during the day. The station is called Dún Laoghaire Mallin and sits right in the town centre, a five-minute walk from the piers.
Dublin Bus routes also serve the town from the city. By car from Dublin city centre it's about 30 minutes depending on traffic — parking around the harbour area fills quickly on weekends.
For arrivals by sea, the Stena Line ferry from Holyhead in Wales docks at the ferry terminal on the north side of the harbour. The crossing takes about 2 hours on the high-speed service.