Eoghan Hi Eoghan, bunq is a Dutch bank. The Dutch IBAN format has only 18 digits, and companies etc. should recognize it just fine. If they don't, they're actually breaking European law. You can try to contact them to resolve the issue.
We also have a useful topic about how to deal with IBAN discrimination here: https://together.bunq.com/d/23003-experiencing-iban-discrimination-here-s-what-you-should-do And there's a great blog post with more tips and explanation written by Wise here: https://wise.com/gb/blog/iban-discrimination
As a last resort, you can always contact your national authority and complain about a certain company. In Ireland this would be the Central Bank of Ireland and the CCPC. It's a good idea to document as much as possible where the company is infringing on your right to use a foreign IBAN, for example with screenshots from their website or with e-mails/support chat that you exchanged with the company.
There is also an initiative started by Wise and other Fintechs that is also supported by bunq where you can report problems with IBAN discrimination. Visit it here: https://www.acceptmyiban.org
