Brian IBAN discrimination is unfortunately something that happens more or less in every country. Of course it's in bunq's best interest for this to not happen at all, but they're also not operating at a scale where they can really change much here.
They have created a short article about what you can do: https://together.bunq.com/d/23003-experiencing-iban-discrimination-here-s-what-you-should-do In Ireland you probably want to complain to either:
As IBAN discrimination is also anti-competitive behavior, sometimes you can have some success from that angle.
As this is a problem for all Europe-wide operating banks, there have been some campaigns started like: https://www.acceptmyiban.org I would also advise you to use their form, too, in addition to your local authorities.