Remove the corned beef from its packaging and rinse it under cold water to remove excess brine.
Place the brisket in the Instant Pot, fat side up.
Sprinkle the spice packet (or pickling spices) over the beef.
Pour in beef broth or water until the brisket is mostly covered.
Add the onion and garlic.
Lock the lid and set the Instant Pot to HIGH pressure for 90 minutes.
Once done, allow a natural release for 15 minutes, then quick release any remaining pressure.
Take the corned beef out and let it rest on a cutting board.
Add potatoes, carrots, and cabbage to the Instant Pot.
Set to HIGH pressure for 5 minutes, then quick release.
Slice the corned beef against the grain for the most tender bites.
Serve with the vegetables and a ladle of broth for extra flavor.