Just a few to help people out if looking for certain things.
Please feel free to add to the list or to comment on the stores that are mentioned. What you thought about them and what you were able to get there.
K & M Evans Art Supplies - it's just off Capel st. at 28 St. Mary's Abbey it's along the Luas line. This place is brillant for nearly everything!!! This is where a lot of teachers that there stuff for school. Paper, card, plastacine, modelling clay, pipe cleaners, pencils etc etc. This is the cheapest place I found when in Art college. And the staff are really helpful.
There's O'Sullivan's Graphi Supplies just off Camden St. 23-25 Grantham Street. This place is great for getting display card for exhibitions etc. So much stuff in this store. Kinda pricey though. Also has a good selection of portfolios, basic and professional, if your heading to art college or want one to show off your work. Also millions of pens and paints etc. I'd shop around for the smaller things you need first. It's more graphic based so not a lot of make 'n do items. I'm pretty sure you can get a discount on certain days if you're a student. [Mod update Oct. 2016: O'Sullivan's is no longer open!
Reads on Nassau Street is great for things like card, glue, pens, pencils, plastic pockets, paint which would be better priced than Eason's RIP OFF CITY!!!! Also good for photocopying cheaply. It's also reasonable for getting business cards made up.
Star School & Office Supplies - There's a few : Artane, Ballymount, Blackrock, Dun Laoghaire and Sandymount. They usually give good discounts to students and are good for paint, pencils, oil pastels, sketchpads, notebooks etc.
Art & Hobby Shop - There's loads scattered around the country. I, myself, never buy much from them. Too overpriced for my liking but they have stuff like Face & body paint which is cool and different art kits.
Okay, so there is a few of the shops I frequented when in Art college. Hope it helps you find what you want. I wouldn't really recommend Eason's at all. It's overpriced and the staff, normally, haven't a clue about the products. If you can't find what you want in the stores I mentioned, ask them in the shop and they may help ya with what you're looking for. I find the staff in most of them really helpful.
Also, check out charity shops for unusual materials use in your art/modelling/jewellery/fashion.