Put the pork loin or roast in a gallon plastic bag along with the packet of taco seasoning. You could also use your favorite Mexican spices instead of premixed seasoning. Marinate for 2 hours in the refrigerator.
Place the pork into a crock pot. Cook on low for 6 hours, or on high for 3 hours.
When the pork is fork tender, pull it apart well in the crock pot and continue to cook for another 30 minutes.
Warm tortillas between paper towels in the microwave. Top tortilla with pork, chopped onion and chopped cilantro.