I love Mexican food and I have eaten in many Mexican restaurants in Barcelona but I think La Taqueria is the best. It is also recommended by many Mexican friends of mine. They say that they make the food as you would eat in Mexico.
With its flowered tables with plastic plates, definitely it is not a luxurious place but has the delicious food with nice Latin music.
They have the menu both in English and Spanish but to be honest it is a bit difficult to understand it even in Spanish :) It is always better to ask the definition of the plate before ordering because sometimes names can be misleading and the guys who work there are really nice and helpful.
If you think about going Friday or Saturday, you must reserve a table as there are only 10 tables in the restaurant. The good thing is you can also make the reservation through their Facebook page. Don’t worry, they really answer fast :)
We went 6 people so I could take many pictures of many different plates but unfortunately I forgot their names. At the end eveveryone was happy with their food so I think the names are not very important as all plates are delicious.
If you are in Barcelona as a tourist and if you like Mexican food, this can be your place to stop for a lunch after visiting Sagrada Familia as it is just around it.
Isileta 
I wish I caught some Mexican food in my trip to Barcelona! Sounds delicious!
I hope you can do it next time ;)