Insert the rack into the Instant Pot.
Add the onion and garlic to the 6-quart Instant Pot distributing it evenly on the bottom.
Remove the Corned Beef Brisket from the package, reserve the spice packet and rinse the meat thoroughly.
Place the brisket in the Instant Pot on the lift-out rack (if there is a fattier side put that facing down) and sprinkle the seasonings evenly on top of the brisket and around it in the pot.
Pour in the beer around the edges of the Instant Pot (to keep the spices on the brisket in place).
Add the lid to the Instant Pot, select MANUAL High Pressure (default pressure setting) and 90 minutes.
When done, do a QUICK RELEASE of the pressure, remove the brisket, brush the seasoning from the top of the brisket (more difficult to remove when it has cooled) and allow to rest.
Add the potatoes (on the bottom) and carrots (on top of the potatoes) to the Instant Pot.
Seal and select MANUAL for 3 minutes on High Pressure.
When done do a QUICK RELEASE and remove vegetables with a slotted spoon.