As others have stated, UK citizens can enter the Ireland without any passport formalities (which is why there is an open border between Ireland and Northern Ireland).
Also, as has been pointed out, most carriers require some form of photographic ID. All airlines do, but individual rules vary. (Ryanair, for instance, will only accept a passport or driving licence; some other operators accept things like bus passes and student ID cards).
Ferry companies are not obliged to insist upon photo ID but some do. I assume that you'll be travelling via Holyhead, which means that you'll be travelling with either Irish Ferries or Stena Line.
Irish Ferries insist upon ID, but it does not have to be in photographic form. Bank card or utility bills (or similar), in the name of the traveller, will suffice:
http://www2.irishferries.com/system/webtest/vi ew/selfservice/controller.jsp?CONFIGURATION=10 08&PARTITION_ID=1&CMD=STARTPAGE&USERTYPE=1
Stena Line simply states that British citizens "are advised to take a form of identity":
http://www.stenaline.co.uk/ferry/faqs/passport s-and-visas/im-a-british-citizen-do-i-need-a-p assport/
It's likely that Stena would accept the same types of ID as Irish Ferries but it might be best to phone them to check: 00 353 818 300 400 or send an email to
[email protected]
Chris