Combine all of the spice rub ingredients in a small bowl. Rub all over the pork shoulder and place the seasoned pork shoulder into the bottom of a slow cooker. Add onion and garlic. Add the bottles of Guinness. Cover and cook on low for 8 hours.
Once the pork has finished cooking remove it from the slow cooker and shred it using two forks. Discard the bones. Return the shredded meat to the slow cooker and mixed it with the juices.
Combine all of the ingredients for the cream in a food processor or blender and pulse for a few seconds.
Serve meat on corn tortillas with cheese, cabbage, and crema.