Preheat your oven to 350 degrees and put dutch oven inside with lid so it can preheat too.
Wash and pierce all around potatoes with a fork. Place corned beef in middle of pot with potatoes around it, or underneath meat used as a trivet (but keep in mind your meat needs to be submerged mostly).
Slice your onion into large chunks, sprinkle around meat. Cut carrots into large chunks, put with onion on outside and top of meat.
Pour your can of beer on top of your meat. 24 oz. is about right. Enough to mostly submerge your piece of meat. Any brand but dark beer will really change the flavor of the beef. Sprinkle your spice packet on top of your meat.
Put lid on and bake at 350 degrees for 2 hours. Then flip corned beef brisket and cook for an additional 30 min at 350 degrees.
Cut end off cabbage and cut it into 4 equal chunks.
Take dutch oven out and put chunks of cabbage inside, cook for last remaining 30 minutes at 350 degrees. If you want your cabbage to crisp up a bit on edges you can leave lid off
Take out, allow meat to rest for 5-10 minutes before slicing. Enjoy!