How can I retain my UK iPhone and SIM when moving to Ireland for temporary basis?

Your options are to either change country on the same account, or to use two accounts. As stated earlier, you cannot have one account set to two countries at the same time.

If you create a second account, you will need a separate email address for that. The same email address cannot be used for two accounts at one time. Also note that it isn't easy to transfer items between accounts so only do this if you really want to have a different persona. If I were doing this using two accounts I would create a new account set in the UK (and email address) that you don't mind getting rid of in a few months and keep your established account for Ireland.

You may also wish to carefully read this document so you fully understand what is required and what happens when you change account countries. Change your Apple Account country or region - Apple Support

They might or might not be affected. Any apps actually left on the device should continue to work unless the bank itself refuses connections from out of country (I've never seen anybody say they had that issue). However, if the old versions stop working and you have to update, you may find that the bank does not bother to distribute its apps out of country. If they shut out the old version and you cannot update, you're stuck. That is a common issue on the forums here—somebody needs to use a banking app from another country, but cannot find the app in the store for the new country. That's how you using two accounts, each on its own device, can help with this. Your UK phone will continue to work with the UK store. Note though, I would not recommend dong that long term. Apple is pretty sensitive to "account abuse" and you are only supposed to be using an account set to a country where you are actually resident (and no, Apple does not recognize dual residency). When you do ultimately cut off your UK ties and your only country is Ireland, you may well find that UK apps (particularly something country specific such as a bank) will no longer update. Eventually your bank will require you be using the latest app version for security reasons and if you can't update your version then that is it as far as app access.

Note that I am glossing over some potential issues but I am going to avoid rambling on here. ;-)

You can change the country, but each Apple Account can only be registered as being used in one at any given time.

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