Northern Ireland is and always has been part of the United Kingdom.

It certainly has not. There was no United Kingdom of any sort prior to James I and VI. There was no United Kingdom of Great Britain prior to the Act of (Scottish) Union in 1707. There was no United Kingdom that included any part of Ireland prior to the Act of (Irish) Union in 1800. The present "United Kingdom of Great Britain and Northern Ireland" has only existed since the Government of Ireland (Partition) Act in 1920.