You can create your own custom regions in OOTP. So for example, you could create a UK Region that does include Ireland. It's a bit tedious, but it's exactly why the ability to create regions was added to OOTP, so users could create their own custom regions/worlds.
Here's a link -
http://www.ootpdevelopments.com/boar...ref-guide.html
I recommend using notepad++ - it's free and perfect for editing the World.XML
You could also do it in one easy step and just edit the current UK Region already built in to include Ireland.