If there is a ‘no deal’ Brexit, Green Cards will not be required for driving in the UK, including Northern Ireland, if you have a valid Irish insurance disc. The insurance disc will serve as proof of insurance to the legal authorities in the UK. No separate Green Card will be required.
A Green Card is an internationally recognised insurance document which demonstrates to law enforcement agencies that valid motor insurance is in place.
UK registered vehicles will still require a Green Card if they plan on visiting European Union countries, including Ireland after a ‘no deal’ Brexit.
Source: The Motor Insurers’ Bureau of Ireland