In a large slow cooker, place the corned beef on the bottom (do not rinse; allow all the juices from the packaging to go inside the slow cooker).
Layer with carrots, onions and potatoes. Top with sprigs of thyme.
Pour the beer and water over the ingredients.
Cook on low for 7-8 hours, or on high for 3-4 hours.
Once the corned beef and vegetables have cooked, move around the vegetables to make room for the cabbage. Submerge the cabbage in as much liquid as possible, place the lid back on the slow cooker and cook on high for 1 hour.
Slice the corned beef against the grain and serve with a grainy mustard.