Each account can only be set to one region. According to the knowledgebase article below, one should be able to change the region on an account. Since this process doesn't seem to have worked properly in your case, I'd suggest contacting iTunes support.
http://support.apple.com/kb/ht1311
However, you could have two iTunes accounts; a UK account and an Irish account. The advantage here is that you don't have to worry about a previous UK purchase not being available in the Irish store.
To get an iTunes account in a country in which you do not reside (and therefore do not have an address or credit card from that country), you could purchase an iTunes gift card from the country in question and follow the instructions in the
How to redeem an iTunes Gift Card on a computer > I do not have an iTunes Store account
section of the following knowledgebase article:
http://support.apple.com/kb/ht1574
You will need to authorize both accounts in the Store menu in iTunes. Once both accounts are authorized, you should be able to update the apps you purchased with your UK account as well as any new apps you purchase with your Irish account.
On your iOS devices, however, you can only be logged in to one iTunes account at a time (Settings app > Store > Apple ID). On the device, over the air, you can only update the apps associated with the currently logged in account.
I hope this is helpful.
One option would be the manual selection of apps for each device.
So instead of syncing all apps with all devices, you could select here which apps to install/remove from each device
Best Answer
1) Nothing happens to your contacts or call making functionality. You will still be able to make calls, and this does not affect calling rates at all.
2) Yes, progress in old apps will be kept. No data on the phone is deleted.
3) No, existing App Store subscriptions will not be cancelled when you sign out.