Right Price Tiles on the longmile road (think they are in Bray also) are cheap and cheerful and have a good selection.Also found Chadwicks Bathrooms & Nationwide Tiles in Sandyford Industrial Estate had a good choice and have some reasonable prices (though not as cheap as right price).
It depends what kind of tiles you are after tbh - ceramic are a good bit cheaper than porcelain.
2 tips when shopping for tiles:
1.Don't buy the grout/adhesive from the tile shop itself as you will get fleeced - buy in a builders providers e.g. brooks,b&q,dpl etc.
2.Check if they will take back unopened boxes of tiles - some places tend to sell you more than you need so this covers you for this senario.