For the instant pot, you will want to use a 4-pound beef. The brine can stay the same.
Transfer the corned beef to the Instant Pot, and add 6 cups of unsalted beef broth. Stir in 1 1/2 tablespoons reserved seasonings.
Seal the lid and select the Meat/Stew or Manual setting on your pressure cooker. Set the cooking time for 55 minutes at high pressure. (The pot will take about 20 minutes to come up to pressure before the cooking program starts.)
When the cooking ends, let the pressure release naturally completely. This will take about 40 minutes. Transfer the beef to a cutting board and tent it with foil.
If adding vegetables: wear heat-resistant oven mitts, and carefully lift out the inner pot and pour out all but 1 cup of the cooking liquid (or 1 1/2 cups if using an 8-quart IP) and return the pot.
Place a wire mesh steamer basket inside the pot. Place the carrots and potatoes in the steamer basket, and layer the cabbage on top. It’s okay if the vegetables are slightly above the 2/3 “max fill” marking, but they should not be above the rim of the pot where they could block the pressure valve mechanisms in the pressure cooker lid.
Secure the lid and set the pressure release valve to its sealing position. Cancel the cooking program, then select the Steam program for 4 minutes at high pressure.
The pot will take about 15 minutes to come up to pressure before the cooking program starts.
When the cooking program ends, perform a quick pressure release by moving the pressure release valve to its venting position right away, taking care not to get burned by the steam.