I have a Canadian passport. I entered the UK with it, and I want to take a day-trip to Ireland.
Will my passport be checked, if so, will it be stamped at both ports (going by ferry)?
If travelling by car or bus, only spot checks are carried out; the same applies if travelling on Stena Line Holyhead-Dublin.
If taking Irish Ferries Holyhead-Dublin on foot, your passport will definitely be checked, and is also supposed to be stamped (as Ireland doesn't "accept" British leave to enter, although the opposite does apply), although in practice they're frequently lazy about it - normally I'd advise you to ask that they stamp it, but if it's only a day trip you're better off not bothering.