For popular CDs,
Welcome to BangCd is best. It's cheaper than CD-Wow and delivery is quicker. It's a similar style of operation to CD-Wow, but based in Singapore.
For more obscure things, I buy from
BlahDVD.com: Shop online for cheap DVDs, CDs, Games, Books, Music Downloads & MP3 Players or from Amazon Marketplace sellers (particularly Import CD Specialists or Caiman). I never buy directly from Amazon because it always works out too expensive.
A useful site for locating CDs and comparing prices is
Cheap CDs, but you will have to make allowance for some suppliers having different delivery rates to Ireland.