Combine corned beef, spice packet of pickling spice and water (16 cups) to cover in a large casserole. Bring to a boil, covered, over high heat. Reduce heat; simmer, covered, 3 hours. Add potatoes and simmer, covered, 15 minutes. Add cabbage and simmer 15 minutes more, or until meat and vegetables are tender.
To make horseradish sauce, combine sour cream, horseradish, mustard, and cayenne in a bowl.
To serve Slice corned beef against the grain. Transfer to a serving dish and surround with potatoes, halved, and cabbage wedges. Garnish with parsley and serve with horseradish sauce and carrots, if desired.
Notes
Tip: If corned beef isn't sold with a spice packet, add 1 tablespoon pickling spice.