I find Second hand/Bargain bookshops a good place to look rather than the main shops (H&figgis,waterstones & Easons)
Try chapters on Abbey street, the winding stair beside the halfpenny bridge (Northside)
Paul recently mentioned a bookshop beside the Dome restaurent in the Stephens Green centre.
There is also a small bookshop in the Georges arcade-got my Craig's Dublin 1660-1860 there -strange place, It's almost as if the owner didn't like to sell his books
You might find something in the Oxfam shop in Parliament street also
Not forgetting the RIAI Bookshop in 8 Merrion square- not very inviting as bookshops go, but worth a visit.(mostly architecture books)