I found this recipe on All Recipes many years ago. I've changed the temp and cooking time and it turns out better. I have not made corned beef any other way since first trying it this way. The sugar helps to cut the saltiness of the corned beef and the beer adds a great flavor.
Preheat oven to 350 degrees F. Rinse the beef completely in water and pat dry.
Place the brisket on rack in a roasting pan or Dutch oven. Rub the brown sugar on the corned beef to coat entire beef, including the bottom. Pour the bottle of Guinness beer around, and gently over the beef to wet the sugar. Sprinkle the seasoning packet over top.
Cover, and place in preheated oven. Bake for 3 hours. Allow to rest 5 minutes. The last hour you can add carrots, red potatoes, and cabbage wedges. Cover with foil and bake 1 more hour.
Note: I find some corned beef takes longer to get tender so just cook until it is done.
Over the years, I have found this recipe works well in the slow cooker. I have never made it on a rack, as the original recipe states.