Place the corned beef in a large pot and cover with boiling water. Bring to a boil, lower the heat, cover partially, and simmer as slowly as possible until very tender when tested with a fork, about 3 hours.
Preheat the oven to 350°F.
Mix the marmalade, mustard, and sugar together in a small bowl.
When the meat is done, remove it from the pot and drain. Place the meat in a baking pan and pour the marmalade mixture over it, coating thoroughly.
Bake until the glaze is crisp and brown, 30 minutes. Serve hot or at room temperature.