Arrange cabbage over corned beef, cover, and continue cooking until cabbage is tender, 45 min to 1 hour (or 1 1/2 to 2 hours on low). Thinly slice the corned beef against the grain and serve with the vegetables and cooking liquid with the horseradish sauce.
Cut the corned beef into 2-3 chunks so it cooks faster and more evenly. Add to the insert of a 6 quart Instant Pot with the fat cap up. Layer with the onion, spices, thyme and beer. Cook on HIGH pressure for 90 minutes then do a quick release.
Add the potatoes, carrots and cabbage to the insert, close, and set to HIGH pressure for another 5 minutes. Natural release for 5 minutes, cut, and serve.
Mix the sour cream and horseradish together in a small bowl. Refrigerate for up to 1 week.