If it's not locked to any us network then you should be able to just get a sim and use it.
If it is locked then you need to get it unlocked first.
iPhones can need activation on a new network.
This process should happen automatically or during an iTunes sync and automatically sends a once off UK SMS which costs 25c.
For this reason you should stick a small amount of cash credit on your account in case this activation process is needed.
You will also need to configure your internet/APN settings.
See here:
http://community.48months.ie/t5/Need-Help/Internet/m-p/34493#M12560Hope this helps.