...almost certainly eBay. Do an advanced search with your Nokia model no. and the word "battery", and search for items "available to the UK". When you find a good price, email the seller and ask them if they'll post to Ireland (see thread).
this may be obvious, and you may have thought about it already: I upgraded recently for EUR 29, whereas a new battery for the old phone would have been 45 - the choice was easy.