Currently we have Category:Ireland (as in the Republic of) and Category:Northern Ireland.
According to the standards we've been using in other countries, a province should just be named like Category:Ulster. It would be a subcategory of Northern Ireland.
The one county that's listed now, County Antrim, could also be a subcategory of Ulster. (And actually, it's name should probably be changed to either Antrim or County Antrim, instead of County Antrim, Northern Ireland, to be consistent with the English and Scottish counties.)
So here's what you should do: Change the category on the profile page to [[Category:Ulster]]. Click on the red link. That will take you to the category edit page, where you can enter [[Category:Northern Ireland]] and save the page.
Let me know if you need any more help! And thanks for helping to populate the categories! :)