[user104658]

There does seem to be a lot of confusion with this. Especially with people using the terms "Great Britain" and "The UK" interchangeably. Great Britain is a geographical term for the larger island of the British Isles. It does NOT include Northern Ireland. Which actually makes "Brexit" a misnomer.

Republic of Ireland IS part of "The British Isles" but that is a geographical term and has nothing to do with politics. It is not part of the UK.

Northern Ireland is part of The UK, but not part of Great Britain, which should be evident from the full name: "The United Kingdom of Great Britain and Northern Ireland".