Add onions, garlic, and bay leaves to Instant Pot and pour the water and broth on top. Place the trivet that came with the Instant Pot inside the container handles up for easy removal.
Place meat flat-side down on trivet. Season with salt and pepper to taste, then add the seasoning packet that came with the meat.
Add lid, lock into place. Set to "Sealing" and select the"Manual" button. Adjust to high setting, and set cook time to 85 minutes.
When cook time is complete, allow pressure to release naturally for 10 minutes before doing a quick release on any remaining pressure.
Remove brisket using trivet handles and potholders, ad transfer to a serving platter. Set aside and keep warm.
Return trivet to Instant Pot, and add carrots, potatoes, and cabbage on top. Cover and lock lid into place. Set to vent to 'Sealing" and select the "Manual" button. Set the time for 3 minutes.
Once finished, allow pressure to release "Naturally" for 5 minutes before doing a quick release on remaining pressure.
Transfer cooked vegetables to serving platter with the corned beef. Slice beef against the grain and serve immediately.