I presume that it's still locked to Vodafone?
You can ask them to unlock it but it may cost you depending on whether or not it's out of contract or you have bought sufficient pay as you go credit over the relevant period of time:
https://nac.vodafone.ie/
Otherwise you can get it unlocked unofficially elsewhere but make sure that you only use a reliable service and make sure that they unlock it properly and not just jailbreak it which can cause problems later on...
Hope this helps.
