hi there Zapatista,
there are actually 2 foreign language bookshops in the city centre :
Foreign Languages ( I think this is the name of it ) is on the street just off Pearse Street. I don't remember the name, but it is the street where you enter/exit Pearse Street train station. The bookshop is a black front and is about 10 doors up from the entrance to the train station.
There is also International Books, on South Frederick Street , which I personally think is better, and they seem to have a better stock of stuff.
If you are just looking for general stuff ( tapes , or complete courses ) Easons and Hodges Figgis also have a very good selection, and can often be cheaper than the two shops above.
For tapes I would recomment the Linguaphone AllTalk, however they are a bit expensive ( they come in 4,8 and 16 hour courses, and prices seem to go from about 40 to 160 euro ) , but you can get them a lot cheaper on ebay.
Hope this helps !