Because I already have an Orange SIM and from what I can tell, it's 5 euro for 250MB for data in the UK and 150 texts. Plus people already have this number.

"UK = mobile internet on your phone within the UK."

UK includes Ireland so it should work fine.

On the O2 site, I don't even see an option to add data.

I asked this question because I wasn't sure. No need to be rude in your reply. it was a genuine question as I've never been to Ireland. I will check out 3 though.