In particular cases where 00 is a valid check digit, 97 will not be a valid check digit, likewise if 01 is a valid check digit, 98 will not be a valid check digit, similarly with 02 and 99.
Banks in Northern Ireland can be part of either the British or the Irish clearing system, depending on their country of registration. Sort code is a bank code used in British banking industry to route money transfers between banks within United Kingdom. IBAN and BIC codes are widely used means of identifying bank account numbers, but not the only international system.