Speak to 02 uk customer services on 4445 to have your account sorted for roaming or buy a local sim while your there and use that. Your phone must be unlocked to all networks.
Ensure your phone is not set on automatic in selecting networks too or it could choose the Ireland version of 02 and be expensive.
Here's a link ref travel.
http://www.o2.co.uk/international/o2-travel