In the base of the slow cooker, use a fork to mash together the pork marinade: achiote paste, garlic powder, onion powder, vinegar, salt, pepper and orange juice.
Add pork to the slow cooker and rotate with tongs until pork is evenly coated with marinade.
Pour in beer and cover with lid.
Cook on Low for 8 hours or High for 4 hours.
While pork is cooking, prepare the pickled red onions by combining all ingredients in a small bowl and tossing to combine. Let sit at room temperature for 30 minutes or in the refrigerator for up to 3 days. Set aside until you're ready to serve.
Remove pork from the slow cooker and let rest until it's cooled down enough to handle. Use your fingers or two forks to shred pork, removing any large pieces of fat.
To remove fat even further from the dish, pour out all of the liquid that is remaining in the slow cooker into a measuring cup with a fat separator. Let the fat come to the top and discard.
Add shredded pork back into the slow cooker and pour in the remaining broth that has been separated.
Serve pork in corn tortillas and garnish with reserved pickled red onions and minced cilantro.
Achiote paste can be found at a Mexican grocery store or online at Amazon.Gloria uses fresh onions and garlic but I prefer to use powder when using the slow cooker as I find it sometimes results in a bitter taste otherwise.