In Article 215 of Directive 2006/112/EC, the following paragraph is added:
‘The prefix “XI” shall be used for Northern Ireland’.
Source ec.europa.eu
See also EU VAT Directive 2006/112/EU & Implementing Regulation 282/2011 – Current versions. This link also incuded a verrion of the EU VAT Directive as applicable per Jan 1, 2021 (with the changes due to the E-Commerce Directive only being effective July 1, 2021)