Combine first 3 ingredients in a 6-quart pressure cooker.
Cover with lid, and seal securely; place pressure control over vent tube. Cook over high heat until pressure control rocks back and forth quickly. Reduce heat until pressure control rocks occasionally; cook 50 minutes.
Remove from heat, run cold water over cooker to reduce pressure. Carefully remove lid.
Remove corned beef; keep warm. Add potatoes and cabbage to cooker. Cook as previously directed 5 minutes. Arrange potatoes and cabbage around corned beef.
Combine sour cream and horseradish; serve with corned beef, potatoes, and cabbage.