Comment by outop

Did you read the headline of that article?

You need government ID to get on a domestic flight in the UK. You also need government ID to get on a flight from the UK to Ireland.

As with the sea border and the land border, this completely invalidates your claim about what ID is required to travel between the UK and the Republic of Ireland.

You don't appear to have travelled between the UK and the Republic of Ireland, ever, or to have flown domestically in the UK since 9/11. You stated above that "they do not check ID on UK domestic flights", not "the CAA does not require ID but all airlines do". The first statement is untrue. Not sure why you are making stuff up in support of an urban legend about the UK/Irish border.

Even if there was a difference between the ID required to board a flight from the UK to the RoI and the ID required to board a UK domestic flight (there isn't - both require govt ID, not necessarily a passport), the situation at the boat and at the land border completely disproves your original claim.