Dry the corned beef brisket with paper towels. Season the corned beef with the seasonings from the packet, pressing lightly to adhere.
Add onion, garlic, thyme, bay leaves, and beef stock with salt and pepper to the insert of the Instant pot. Place a rack on top of the onion and beef stock mixture, then add the seasoned corned beef. Pour the Guinness over top of the beef.
Seal the lid and set the unit to HIGH pressure for 1 hour and 40 minutes. Cook. When the time is up, let the pressure release naturally for 20 minutes, then manually release the remaining pressure. Remove the corned beef and place it on a dish or a cutting board, spooning a bit of the cooking liquid over the top. Cover with aluminum foil to keep warm.
Add the carrots, and potatoes straight into the cooking liquid, then place the cabbage over top. Season with lots of salt and pepper. Seal the lid and set the unit to HIGH pressure for 3 minutes, depending on how soft you like your vegetables. Quick release the pressure.
In a small bowl, make the optional butter sauce. Melt the butter in the microwave, then add minced parsley, and a pinch of salt and pepper to taste. Mix together and spoon it over the veggies!
Slice corned beef against the grain. Spoon a little bit of the reserved cooking liquid over the corned beef slices. Drizzle the veggies with the melted butter sauce and more parsley if desired. Serve with stone ground mustard. Enjoy!
Store leftovers in an airtight container in the fridge for 4-5 days. To reheat, you can use a microwave, an oven, or a stovetop. Add a bit of broth or water to keep the meat moist and prevent it from drying out. To freeze, all the leftovers to cool at room temperature before transferring them to a freezer-safe bag and freeze for up to 3 months. Thaw in the fridge overnight, then reheat.