I was recently told* about Boojum, a Mexican restaurant on Millenium Walkway (opposite Temple Bar). It's nothing fancy but it's good: quick, cheap, and tasty. Typically it'll cost roughly €7; service is from a counter, with the food being prepared to order in front of the customer.
Not quite a match for Pablo Picante but it comes close - and it has the advantage of having much more seating space, including outdoor seating. Recommended.
*My thanks to newly launched blog, Fluff and Fripperies.