Rinse corned beef brisket under cold water to remove excess salt.
Place the trivet inside the Instant Pot and add onions and garlic at the bottom.
Set the corned beef on top of the trivet, sprinkle the included spice packet evenly.
Pour beef broth into the pot until it almost covers the meat.
Seal the Instant Pot lid and cook on High Pressure for 90 minutes.
Allow the pressure to release naturally for 20 minutes before opening the lid.
Remove corned beef and cover loosely with foil to rest.
Add potatoes, carrots, and cabbage to the Instant Pot liquid.
Cook vegetables on High Pressure for 3 minutes, then quick release.
Slice corned beef brisket against the grain and serve with vegetables.