At some time post-pandemic, a family member intends to visit the UK and the Republic of Ireland. While they are a UK citizen by birth, they have been resident in Canada for many years, and only hold a Canadian passport at present.
While visiting the UK, they would like to visit Ireland (by ferry) for a few days. I understand that UK citizens can travel freely in the Common Travel Area, but ferry operator Stena Line recommend bringing some proof of citizenship:
British or Irish citizens travelling on our Irish Sea routes do not need a passport to travel to Britain or Ireland but are advised to take a form of identity. A valid passport, photographic driving license, International Student Card, government issued photo ID card, health insurance/social security photographic card, photographic bus/train pass or EU Citizen Identity Card will usually suffice.
The traveller has a UK birth certificate (short form), but none of the documents listed above. (They could apply for a new UK passport, but would prefer not having to go through the hassle and expense of doing so.)
If they chose to travel on their Canadian passport alone, what procedures or difficulties should they anticipate when sailing from the UK to Ireland?