Identification is not legally necessary for domestic journeys within the United Kingdom.
Take the following three-legged journey:
- Fly from a non-EU country to Dublin
- Ride a coach to Belfast
- Sail a ferry to Liverpool
In this journey I go through passport control in the Republic of Ireland but not in the United Kingdom.
A carrier might demand such documentation for their own purposes but I will ignore this aspect here.
Does this mean that because of the Common Travel Area, and ignoring intelligence-led interdiction, a portion of UK border control is delegated to the Republic of Ireland?