In a small bowl mix together the sour cream, the horseradish and the scallions. Cover and refrigerate for at least 2 hours prior to serving.
Put the corned beef in a Dutch oven. Pour the beer and enough water to cover the meat. Add the bay leaves.
Bring to a boil over high heat. Reduce the heat to low and skim the foam from the surface of the water. Cover and simmer for about 2 ½ hours, or until the corned beef is tender.
Remove from the liquid, wrap in foil and set aside.
Add the potatoes, cabbage, onion and carrots to the pot, cover and cook over a gentle boiling simmer for 20 minutes, or until tender.
Return the corned beef to the pot to warm up, 3 to 4 minutes.
Cut the meat across grain and arrange on plate with the vegetables and a little of the cooking juice.
Serve with the horseradish cream.