If I buy an unlocked iPhone 5 from the US can I use it with Vodafone IE?

If I buy an unlocked iPhone 5 from the US can I use it with Vodafone IE?

I don't see why not.

Yes you can. Unlocked iPhone is defined to be an iPhone which is not tied to any particular network. If you have an unlocked iPhone, definitely you can use it with a network of a compatible provider and insert any compatible SIM card.