Place the corned beef brisket fat-side up in the Instant Pot. Sprinkle the included seasoning packet over the top. Add the quartered onion, smashed garlic, and bay leaves around the meat.
Pour in the water and beer. The liquid should come about halfway up the brisket.
Lock the lid, set the valve to sealing, and cook on high pressure for 90 minutes.
When done, allow a natural pressure release for 15 minutes, then quick release the remaining pressure. Carefully remove the corned beef to a cutting board and tent it loosely with foil to keep warm.
Remove and discard the bay leaves. Add the halved baby potatoes and carrot pieces to the cooking liquid in the pot. Place the cabbage wedges on top.
Lock the lid again, set the valve to sealing, and cook on high pressure for 4 minutes. Quick release the pressure as soon as the cooking time is done.
Slice the corned beef against the grain into thin slices. Serve alongside the potatoes, carrots, and cabbage wedges with a drizzle of the cooking liquid and whole grain mustard on the side.