There are a large variety of ferry routes to and from Ireland. The ferry crossing to Ireland will vary depending on the route you choose and the type of ship. Here I am talking only of ferry route between Ireland and England, Wales, Scotland and the Isle of Man. There are other ferry routes to Ireland from France, Spain and other countries.
From England it is possible to get a ferry to Ireland from Liverpool, Birkenhead and Fleetwood – all in the NorthWest of England. The Liverpool and Birkenhead ferries depart from opposite banks of the river Mersey. From Birkenhead you can get a ferry to Belfast or Dublin; from Liverpool to Dublin; from Fleetwood to Larne.
From Wales you can get a ferry from Swansea, Pembroke and Fishguard – all in south Wales or from Holyhead in north Wales. The routes are Swansea to Cork, Pembroke to Rosslare, Fishguard to Rosslare, Holyhead to Dublin and Holyhead to Dun Laoghaire.
From the Isle of Man ferries go from Douglas to Belfast and Dublin.
From Scotland you can get a ferry from Troon, Cairnryan and Stranraer. The routes are Troon to Larne, Cairnryan to Larne and Stranraer to Belfast.
On all these routes the time and frequency of ferries to Ireland vary throughout the year. The Irish Sea is also known for being highly variable which can also affect ferry timing.