Good question.
Technically Northern Ireland is not part of the mainland or indeed the British Isles as it shares its land mass with the Republic of Ireland which is a totally independent country.
The correct relationship is that it is part of Great Britain, which to give it its proper title The United Kingdom of Great Britain and Northern Ireland.
I suppose only the bank in question could answer whether they consider it as UK mainland as there is always a debate as to whether it is or not.
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