Remove the corned beef from the package and gently rinse to remove excess brine. Set the beef and the spice packet aside.
Place half of the chopped onion into the bottom of the slow cooker and then set the corned beef, fat side up, on top of the onion. Place the remaining portion of the onion around the corned beef.
In a mixing bowl or large measuring cup combine the broth, worcestershire, light brown sugar, salt and pepper, stir to combine this, then pour over the top of the corned beef.
2 cups beef broth, 2 tablespoons worcestershire sauce, ½ cup light brown sugar, 1½ teaspoon salt, ½ teaspoon ground black pepper
Pour the Guinness into the slow cooker over the corned beef. Then sprinkle the packet of pickling spices directly onto the corned beef.
After the meat has cooked for 4-5 hours, add in the sliced carrots, and potatoes. Continue cooking on low.
During the last hour of cooking, add in the quartered cabbage around the meat.
Cook on low for a total of 8-10 hours until tender and meat has reached a temperature of at least 195℉/90℃ - 205℉/96℃. In the 195-200℉ range the meat will be tender and sliceable. In the 200-205℉ range the meat will be more tender and shred. This depends on your preference.