On May 3 1921, Northern Ireland officially came into existence as the division of the island of Ireland became effective legally. The border divided the 32-county island into two separate legal jurisdictions. Six counties in the north-east became Northern Ireland, which is still part of the United Kingdom.
The other 26-county territory became the Irish Free State on 6 December 1922, which was renamed “Ireland” on 29 December 1937. This partition of Ireland had the effect of beginning the process of settling boundary issues between the nationalists (who wanted independence from British rule) and the unionists (who wanted to remain in the United Kingdom).
Northern Ireland was given special status as a result of the 2019 Brexit settlement between the UK and the EU. While England, Scotland and Wales no longer follow EU rules, Northern Ireland still does, because it shares a land border with the Republic of Ireland, an EU member. That’s been problematic since customs checks would take place on goods entering Northern Ireland from elsewhere in the UK.