Will it costs to send an iMessage from England to Ireland and vice versa?

Just wondering my boyfriend and I both have iPhones with iMessage and he is away in Ireland for a couple of days.
Will iMessage still work and be free for us or does he need Wi-Fi for iMessage to send to me?
How will it work out?
He is on 02, I'm on orange uk.

You need to check with your service providers

You are messaging a UK phone based in Ireland, you are not messaging Ireland - you need to say that when you're asking your provider for the rate, the costs of messaging a UK number in the Republic of Ireland are different from messaging an Irish number

Imessages that are sent by one Wi-Fi connected device to another are free. I have an ipod touch that has imessage and of course I'm not charged because it's not connected to anything! But if you were to send a message that wasn't from Wi-Fi you would be using the cell tower.

Yes, it's still free once you're connected to Wi-Fi.