I am not honestly too sure of the answer to this. The only knowledge I have of this comes from my own experience applying to university in Dublin. Unfortunately, I think that your Irish passport may not be very useful at all. I only applied to Trinity College Dublin, so this may not apply to other universities, but I believe that your fee status is not decided by your passport or nationality, but where you have lived and worked in the five years preceding your application. Also, they are more concerned about EU/Non EU than they are about specific countries. So I believe that where you are currently living is more important than your passport. I am not too sure about this though, and it would be best to get in touch, as I may be mistaken!