English is an official EU language because Regulation 1 Article 1 says so [0] and hasn’t been changed. In practice, English is the most widely used language in EU institutions, so it would be have been silly to remove it after Brexit.
English at this point has stopped culturally belonging to the United Kingdom and whilst one can discus it's not so very moral way of getting there, it's become the bridge language for people of different languages to communicate in, further solidified by the internet.
That said, whenever there is a language selection UI (e.g. at banking machines or institutional websites) in wider Europe that uses flags to represent languages -- probably not a good idea to start with, but very common -- the Irish tricolour should be used to indicate English rather than the UK or USA flags. (although cf Airteagal 8 of Bunreacht na hÉireann).
Cool, so the European Union and overlapping institutions could see this as an opportunity to promote greater public knowledge about one of their respective member states. Seems like an argument in favour of encouraging the display of a member state's flag rather than that of a non-member-state or former member state (especially given that state's history with respect to Ireland).
Using flags alone is already poor UI since there are many languages which spill across the borders into multiple member states and non-member states, and some member states with multiple official and commonly spoken languages.
But a menu item that reads: [Irish flag] (English) like one that reads [Swedish flag] (Svenska) does not seem worse than the legacy use of the UK flag or the popular use of the US one.
AFAIK Ireland only listed Gaelic as their official language with UK having English. That caused a bit of a problem during Brexit since technically English wasn't officially an EU language anymore. I guess they resolved it somehow.
