Cut a 5-inch square of cheese cloth and lay it on a flat work surface. Put 4 of the garlic cloves, the thyme, pickling spice, bay leaves, and peppercorns in the center of the cloth. Tie the opposite corners together, creating a pouch.
Put the brisket and the spice pouch in a 12-quart stock pot and add enough cold water to cover the brisket by 4 inches. Bring to a boil over high heat and then reduce the heat to maintain a simmer, cover, and cook until the beef is nearly fork-tender, about 3-1/2 hours.
Add the potatoes, carrots, onions, celery, and cabbage. Simmer until the meat is fork-tender and the vegetables are just tender, about 30 minutes more. With tongs, transfer the meat to a cutting board, tent with foil, and let rest for 15 minutes.
Using a wire skimmer or slotted spoon, transfer the vegetables to a foil-lined baking sheet. Reserve the cooking liquid and discard the spice pouch.
In a 1-quart saucepan, melt the butter over medium-low heat and add the remaining garlic clove. Cook until the garlic is lightly browned, about 5 minutes. Discard the garlic.
Meanwhile, position a rack in the center of the oven and heat the broiler on high. Drizzle the butter over the vegetables and broil until lightly browned, about 5 minutes.
Slice the corned beef and arrange on a serving platter, surrounded by the vegetables. Drizzle some of the cooking liquid over the meat and pass the remaining liquid and malt vinegar (if using) on the side. Serve hot.
Go all out for this fun holiday. Finish off your St. Patrick's Day meal with this stunning Chocolate Irish Whiskey Cake.