The Mexican Restaurant is a large restaurant located in Los Santos, San Andreas. As implied by its incredibly mundane, uncreative name, it specializes in Mexican food. It is inaccessible to the player and plays no role in the storyline what so ever.
Redsands East in Las Venturas has a "The Mexican Restaurant", seemingly inside a similarly generic "Hotel". It is located across the diagonal from Las Venturas Fire Station No. 3, above the Harry Gold Parkway.