How long have you lived in Southern Ireland? I would go with which ever regiment is your local regiment/local to the area you've lived in the longest, have things in common with the other people in your regiment is always a bonus.
I believe the Mercian Regiment recruit from Birmingham as do the RRF, Irish Guards, Grenadier Guards but I'm not sure about R Irish.
If you're that way inclined you could also go for Para Regt as they recruit from all over the country.
What do you want from your career? Do you want to be armoured, mechanised or light infantry?
If you want peoples advice and what regiments you should think about joining then we'll need a bit more info, but remember ultimately we can only advise from our own experience and the final choice will be down to you.