there is no 'duty free' as such when travelling within europe. it is possible to shop in the 'duty free 'shops though - is it something in particular you are wanting to buy?
Agree with Ethel. Most things are as expensive or more expensive in Airport Shops these days and as has been said there is no duty free any more when flying within the EU.
I believe cigarettes and tobacco are cheaper, but seeing as I have never looked myself recently I may not be correct.
Cheaper to buy perfume in places such as 'The Perfume Shop' or from one of the online sites.
As others have indicated, there's no 'duty free' entitlement when travelling within the EU. Your daughter can bring back as many 'duty paid' goods as she likes but she's unlikely to save anything. In particular, cigarettes are dearer in Ireland than in the UK: http://www.the-tma.org.uk/page.aspx?page_id=35