This is a really great online Irish bookstore. It sells new and secondhand books, the prices are quite competitive and the site boasts free shipping worldwide.
2. Amazon
If you can't find a book anywhere else, then Amazon is your best bet. Though, prices for shipping do bulk up the costs.
3. Buy and sell
The used books section on the buy and sell website is another great option and from time to time, school books are posted here from time to time.
4. Oxfam
Secondhand shops are great places to pick up second hand books cheaply, particularly classic novels.