If you want to take the ferry to the UK or France, Irish ferries go from Dublin to Holyhead (UK) or Cherbourg (France) and from Rosslare in County Wexford ferries go to Pembroke, Roscoff, Cherbourg and Fishguard Port (UK). You can also go from Belfast (Northern Ireland) to Liverpool or Cairnryan (UK).
If you leave it to last minute booking, the fare may be expensive. Keep an eye on the website because fares fluctuate. So if your plan is to travel from Ireland to the UK or France, book in advance!
Note: Ferry prices work similar to Aeroplane prices. All ferry departures are on the East coast.