Remove roast from the brine, rinse if desired, then place corned beef onto the trivet inside your Instant Pot.
Cover with a mixture of two cups brining liquid and four cups fresh water.
Add the lid, seal the venting knob, and set to high pressure for 55 minutes.
Do a quick release, carefully open the lid once all pressure has released, then add in whole red potatoes and carrots.
Seal the lid and cook on high pressure for 5-7 minutes (depending on the size of your potatoes).
Do a quick release again, carefully open the lid once all pressure has released, then add in the cabbage wedges.
Seal the lid one more time and cook on high pressure for 2 minutes.
Quick-release the pressure, allow to cool slightly, then serve slices of the meat in a bowl with potatoes, carrots, cabbage, and more brine as desired.