Preheat the oven to 300 degrees F (150 degrees C). Line a 9x13 inch roasting pan with aluminum foil, leaving enough extra extending over the sides to cover and seal in the roast.
Rinse the brisket, and pat dry. Rub with pickling spice, and place in the prepared roasting pan. Arrange celery, orange and onion slices on and around the roast. Pour in 1/2 cup of water, and wrap aluminum foil up over the roast tightly, making sure the ends are sealed.
Bake for about 4 hours in the preheated oven, or until meat is tender.
About 45 minutes before the roasts time is up, heat 3 tablespoons of margarine and 1/4 cup of water in a large pot. Add cabbage and apples, cover, and simmer over low heat for about 30 minutes. Occasionally shake the pot so that nothing sticks to the bottom. Serve with remaining margarine and sliced corned beef.