IOS – Purchases and updates after changing AppleID & Primary Email Address

Home sharing should allow you to see each other's libraries over your home network, and drag apps from your wife's onto yours and vice versa. You can definitely do this despite having two accounts - my wife and I have a similar problem. You may have to enter both passwords from time to time to authorize purchases, though.

I found some help setting this up from "Activating Home Sharing in iTunes".

I believe you can also sort hers by "items I don't have" so you can see what she might have bought of interest to you, although I'm not sure you can automatically sync them.

This SuperUser question seems to suggest that merging two accounts is not an option:
Can I merge 2 iTunes Store accounts?

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.