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I doubt you'll have an issue at Dublin Port. On the two occasions I did this route last year the first time there was a cursory glance at any document with an address on, the second there was no check whatsoever!
Unlike at Dublin airport where you have to go through passport control to get out. Now that would be an interesting one as officially a UK citizen does not require a passport to travel to the Republic of Ireland (although most airlines insist on one for identification purposes). At UK airport passengers on flights from the ROI (evidenced by showing a boarding card) can go through a separate lane to bypass passport control.
Fly from the UK to Dublin several times a year. I always produce my UK Drivers Licence at Passport Control in Dublin Airport, and have no problems entering the Country. Have only once in nearly 30 years been asked to produce my boarding card to verify that my flight originated in the UK. BTW, if flying to Ireland with Aer Lingus or Flybe from the UK you do not need a Passport, your Driving Licence will suffice. Ryanair requires all Passengers to have a valid Passport, (even for UK internal flights). Even when I am coming off a Ryanair flight from the UK I will show my Driving Licence rather than my Passport which stays firmly in my pocket.
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