For IBS sufferers, many ethnic foods can be tricky. Most Mexican restaurants start you out with chips and salsa; the chips are usually corn, which is likely safe, and you may be able to handle mild salsa, but pass on the hot salsa and people who challenge you to a chili pepper eating contest!
Many dishes come with or actually incorporate tortillas, which are generally corn or flour (which means wheat) — make sure you get the corn variety if you have wheat sensitivities.
Cheese can be shaky, but ask what type of cheese they use; if it’s on your safe food list, it may work for you. Lean portions of chicken, beef, and pork are also safe.
Many Mexican menus have complex dishes that have meat, beans, cheese in a wrap covered in sauce and more melted cheese. Such an adventure may not work for you, but fajitas are a great alternative. They often consist of simply grilled meat and vegetables with tortillas and condiments on the side so you can build your own dish.
If you don’t want to be tempted by sour cream, ask for a safer alternative, such as more guacamole or mild salsa. If the flour tortilla doesn’t work for you, ask for romaine or iceberg lettuce leaves to stuff with your fajita mixture, or just use a knife and fork instead.