The new XI and XU country codes were added for use in the Republic of Ireland only (to be selected against customer and supplier accounts in Northern Ireland (XI) and potentially GB (XU).

It should not be possible to select these country codes in Company Details, but they are available and doing so will prevent the user from switching on Online VAT submissions.  

The fact that they are available can cause confusion as Northern Ireland is not part of Great Britain but in order for the system to work correctly, you have to select this option if you are a Northern Irish Company - rather than what would seem like a far more logical United Kingdom (Northern Ireland) option.

Alternatively, you could make the XI code work the same as the GB code if selected in Company Details.