Add garlic, onion, and bay leaves to your Instant Pot and pour in the broth and water. Place the trivet inside with the handles positioned up for easy removal later.
Place the brisket fat-side down on the trivet. Season with salt and black pepper, then sprinkle the included spice packet over the top.
Lock the lid into place, set the vent to “Sealing,” and choose Manual/Pressure Cook on High for 85 minutes.
When the timer goes off, allow the pressure to release naturally for 10 minutes, then do a quick release for any remaining pressure. Using potholders, carefully lift the trivet out by the handles and transfer the brisket to a platter. Keep warm.
Return the trivet to the pot and add the carrots, potatoes, and cabbage on top. Lock the lid, set vent to “Sealing,” and cook on Manual/High for 3 minutes.
Allow pressure to release naturally for 5 minutes, then quick release the rest.
Transfer vegetables to the serving platter with the corned beef. Slice the beef against the grain and serve immediately.