Will an iPhone with a Dual sim from china work in Ireland?

If you unlock it by jailbreaking it will.

Jailbreaking alone does not allow you to use another carrier's SIM card in the iphone. It has to be jailbroken and then unlocked. Once you jailbreak it then Cydia will be on the device and then you go in to Cydia and run Ultrasn0w to unlock it