Place the corned beef in the Instant Pot and sprinkle the spice packet over the top.
Add the onion, garlic, and water or broth to the pot.
Close the lid and set the Instant Pot to pressure cook on high for 90 minutes.
After cooking, allow a natural pressure release for 15 minutes, then quick release the remaining pressure.
Remove the corned beef and cover to keep warm.
Add the cabbage, carrots, and potatoes to the pot, then cook on high pressure for 5 minutes.
Perform a quick release, serve the vegetables with the corned beef, and enjoy!