My sister bought me a £30 gift voucher which is completely unusable

Sorry but gifts, as it says clearly when you go to buy one in the iTunes Store, can be used only in the country of purchase, so a UK gift certificate cannot be used in the Irish iTunes Store. Your sister can probably redirect it to someone in the UK who can use it, even to herself, so she won't have lost the money, but there's no way you will be able to redeem this gift.

Regards.

this helped me out a lot too thank you, but what do you mean by redirect it without loosing the money?

Someone who gave a gift through the iTunes Store can resend it, and at that time can enter in a different email address to send the gift to someone else.

http://support.apple.com/kb/HT1541

Regards.