I can't remember who recommended it (thank you, by the way, I mean to say I finally got down there but couldn't remember the thread!) but there is a Mexican food stall in St George's market on Saturdays, and it is very good (for Belfast standards!). The salsa is yummy, the guacamole is good, he makes his own tortilla chips (though I don't think he makes the tortillas), and you can buy corn tortillas from him (and probably other ingredients). It is totally worth checking out - what I like to do is get some salsa, guac, tortillas and whatever from him, and then some fresh meat from the various butchers in the market, for to make dinner at home later that day. Sawers in the town also has some basics for Mexican food, a step above Old El Paso, but be warned, you'll pay through the nose for it.
But as far as Old El Paso goes, I don't turn my nose up at it because I use it as a springboard for making my own stuff. It helps that my folks from home send me various sundries but even without that you can make your own good Mexican stuff here using OEP as a base.
There used to be a couple "Mexican" restaurants in Belfast, where I had my first enchildadas-in-a-bowl experience, but they all went bust. I haven't heard of any new ones opening but maybe there is?