NEW from 2025 - unless you have a British or Irish passport you MUST pre-register online to cross the land border into Northern Ireland.
You can NOT register on arrival in Northern Ireland.
Permission to travel costs £16 and is valid for multiple entries to the United Kingdom for two years, or until your passport expires.
Individual applications are required for each traveller - INCLUDING BABIES AND CHILDREN.
You should apply by downloading the official UK government phone app, or online - AT LEAST 72 HOURS before crossing into Northern Ireland.
Avoid being ripped off by dodgy sites on Google: use this direct link -
https://www.gov.uk/guidance/apply-for-an-electronic-travel-authorisation-eta
Border Force officers can ask to see your passport whether arriving by car, coach or train.