Layer 1/2 of the onion, carrots, celery and garlic in the bottom of a slow cooker set to low.
Add corned beef brisket, fat side up.
Add remaining onion, carrots, celery and garlic.
In a bowl whisk together the broth, tomato paste, Worcestershire, brown sugar, celery seeds and red pepper. Pour over the beef.
Sprinkle beef with salt and pepper.
Slowly pour beer around the beef and cover the cooker. Cook on low for 8-9 hours until the corned beef is tender.
Remove meat and slice or shred as desired. If storing leftovers, I would add a bit of the sauce from the slow cooker so that the beef doesn't dry out when reheated.