Remove your corned beef brisket from the packaging and set aside the seasoning packet.
You can wash off the brisket if you'd like to, but I just stick it right in the Instant Pot.
Add 4 cups of water to the Instant Pot and sprinkle the contents of the seasoning packet on top of the brisket.
You can add the potatoes, carrots and cabbage to the top now if you don't mind your veggies getting mushy. If you prefer firmer veggies, wait to cook them (this is what I did).
Set the Instant Pot to manual setting for 90 minutes and make sure the vent is set to "Sealing" and not venting and then get your veggies ready while it cooks.
After you've finished cooking a corned beef brisket in the Instant Pot and the steam has been released using the natural release method, open the pot and remove your brisket and set it on a plate.
Add in all your veggies to the leftover juice in the Instant Pot and cook on manual for 5 minutes.
Enjoy your Instant Pot Corned Beef Brisket dinner.