Rinse the corned beef brisket under cold water and pat dry with paper towels. Trim excess fat if desired.
Place the brisket in the slow cooker and add the seasoning packet that comes with it, if included.
Peel and chop potatoes, carrots, and cabbage into large chunks. Arrange them around the brisket in the slow cooker.
Pour enough water over the brisket and vegetables to cover them completely.
Cover the slow cooker and cook on low heat for 8-10 hours or on high heat for 4-6 hours, until the beef is tender and easily pierced with a fork.
Once cooked, remove the brisket from the slow cooker and let it rest for a few minutes before slicing it against the grain.