As one of the main shipping harbours in Europe, it seems very appropriate to travel to Cork by ferry. They sail through the night to Swansea, Wales, and Roscoff, Brittany, so you can arrive at your destination early for a full day's sightseeing.
The Cork - Swansea car ferry leaves six nights-a-week at 9pm from Ringaskiddy, 8 miles south of Cork. Likewise, a ferry leaves Swansea at the same time, travelling in the opposite direction.
There is a special coach service which leaves Cork bus station each evening to the ferry.
Unusually the ferry was crewed entirely by Poles, which made it feel like a proper overseas holiday, though they all spoke impeccable English :-)
A return ticket for a foot passenger was 22 GBP.
Cork has an on again-off again relationship with ferries from here to the UK. For many years we had the Innisfallen and then the rise in air travel caused the fery service to decline in popularity. Eventually the Cork-Swansea service was reinstated but as of this year it is no longer running. We do have an ongoing ferry connection to France though and the Brittany Ferry ship leaves Ringaskiddy every Saturday afternoon and returns the following Friday night. This trip is the quickest between France and Ireland, about four hours shorter than the Rosslare-Cherbourg or Rosslare - Roscoff service. Leaving Roscoff on Friday evening it will get you into Cork by around lunchtime on Saturday. It's a lovely trip, not least the long slow journey up Cork harbour but it is very expensive. Fares for a car and two passengers in peak season could set you back around EUR 700 and it is compulsory to book sleeping acommodation. The cost of cabins really pushes up the price but if you book early you could maybe reserve reclining seats which work out considerably cheaper. It isalso possible to get to the UK and Europe by ferry if you travel to Rosslare or Dublinwhere you can get connections with Irish Ferries or Stena Line.
The ferry from France berths at Ringaskiddy in the outer harbour, about 16km from the city centre. The phon number of the port is included below.