Lunch is always much better value than Dinner in Dublin. For cheaper dinners, go for early bird specials or pre-theatre deals.
Fallon & Byrne on Exchequer Street is a good spot - the upstairs restaurant and downstairs wine bar are quite pricy (although they do good deals), but the deli counter on the ground floor is outstanding - you can buy what you want at the counter, get it heated up and eat it at the cafe, which also does maybe the best coffee in Dublin.
Next door to Fallon & Byrne on Exchequer Street is the Ukiyo Bar which does a great value 10 euro bento box at lunch, and has reasonable priced sushi the rest of the day. If you like Japanese, the two branches of Yamamori Noodles do good value lunch deals - the summer salad meal for 10 euro in the branch on the quays is very good.
Gruel on Dame Street is a long time favourite - it does great rolls at lunchtime and a nice informal menu in the evening up to 8pm - a great spot.
Various branches of Cafe Bar Deli are good for fairly well priced Italian food. For something more informal, the Enoteca delle Langhe is good for a glass or two of wine and some pasta.
In the evening, the Chinese and Korean places on Parnell Street and Capel Street are maybe best value - just pick the one with the biggest crowd, thats probably the one with the decent chef at the moment. My long term favourite in the area is the Hop House/Kimchi restaurant near the junction of Parnell Street East and O'Connell Street (also known as the Shakespeare). Lunch isn't great value, but dinner is, especially if you drink lots of beer or Korean spirits with your dinner.
For a good pre-theatre deal try Chapter One, its michelin starred, at regular restaurant prices.