In a large stock pot or mixing bowl immerse the corned beef in water (covered by a few inches); cover or seal with plastic wrap and place in the refrigerator overnight.
Remove the corned beef, rinse and set aside.
Line the bottom of the slow cooker with the onion wedges, unpeeled carrots and celery. Place the corned beef fat side up on top of the vegetables.
Sprinkle the seasoning packet, bay leaf and thyme sprigs on the corned beef. Pour the beer and water over the corned beef. Place the lid on the slow cooker.
Set slow cooker on low for 10 hours.
After 8 hours: Place the potato wedges and peeled carrots around the top of the slow cooker around or on top of the corned beef. Return lid to slow cooker.
After 9 hours: Place cabbage wedges on top of the vegetables (do not pack the slow cooker too full). Return lid to slow cooker.
When the cooking completes, scoop top vegetables into serving vessels using a slotted spoon to keep cooking liquid in the slow cooker. Remove the corned beef from the slow cooker and allow it to rest for at least 20 minutes before slicing across the grain to serve. Excess fat may be trimmed before serving.
The corned beef may be prepared ahead of time and stored in a container with some of the cooking juices in the refrigerator until serving. For leftover use, discard the cooking liquid and store the corned been in a plastic storage bag or sealable container.