To cook corned beef dishes in a slow cooker, place 2 to 4 pounds of corned beef brisket in a slow cooker, and add a sufficient amount of liquid to cover the beef. Cook the brisket on the low setting for as long as nine hours.
To make basic slow cooker corned beef, pour 2 cups of water and 1/2 cup of white vinegar into a slow cooker, and add 1 peeled, quartered onion. Sprinkle in 1 tablespoon of brown sugar, and add about 2 pounds of corned beef. Cover the slow cooker, and cook the beef on the low setting for seven hours. Top the meat with mustard pickles, relish or chutney.
To make slow cooker corned beef and cabbage, place 1 quartered onion, 4 to 6 quartered red potatoes, 2 halved celery stalks and 4 chopped carrots into the bottom of a slow cooker. Rinse a 4-pound corned beef brisket, and place it on top of the vegetables. Add a 12-ounces of stout or dark ale, 1 tablespoon of pickling spices and enough water to cover the meat. Cook the meat and vegetables on low for eight to nine hours, remove them from the slow cooker, and cover them with foil. Cut 1 medium cabbage into six wedges, and cook it in the slow cooker for 20 to 30 minutes, until softened but still crisp.
Slice the corned beef brisket across the grain. Serve the brisket and vegetables with horseradish sauce and mustard.