I actually did some research on this, looks like it should be:
885(2) TCA 1997 makes it obligatory for various documents, including an invoice, to carry the person's tax reference number.
(2) For the purposes of the Tax Acts and the Capital Gains Tax
Acts, the specified person in relation to a business shall ensure that
the specified person’s tax reference number or, if the specified person
has more than one tax reference number, one of those tax reference
numbers or, if the specified person has no tax reference number,
the specified person’s full names and address is or are stated
on any document (being an invoice, credit note, debit note, receipt,
account, statement of account, voucher or estimate relating to an
amount of £5 or more) issued in the course of that business.