I have an O2 UK PAYG SIM, with their 321 offer (3p pm call, 2p text, 1p per MB data). I have used it sparingly here in Ireland, mostly for texts to UK numbers and the odd data usage. I wasn't charged roaming rates. I must use it at least once every 6 months to keep the credit and number. I don't know if they are still offering the 321 deal.
I got the SIM in Newry a few years ago. I used to top up by buying credit in a shop in NI when visiting, I understand I can top up online too (haven't tried it yet).