When on 20th May 2018 the UK mainland adopted the new rules that allowed classics that reached 40 years of age that had not been substantially changed in the last 30 years to become MOT exempt, Northern Ireland was left behind due to the individual political situation that then applied.
In 2020 the Northern Ireland Government announced it was going to follow the rest of the UK in adopting the MOT exemption and this came into effect on 12th October 2020. The announcement from the Infrastructure Minister Nichola Mallon can be found here
Of note there is no mention of the aspect of significant changes being done over 30 years ago being disregarded for consideration whether the car can claim exemption as it is in the mainland. However, another Northern Ireland Government web page does mention this here https://www.nidirect.gov.uk/articles/mot-scheme
This change is welcome ensuring that all of the UK shares the same rules.