Add corned beef to Instant Pot with 1 cup of water.
Cook on high pressure for 80-90 minutes and release pressure naturally for 10 minutes, and then release remaining pressure.
Add cabbage, carrots and potatoes and shut the lid. Cook for 4 minutes on high pressure with a quick release.
To make this dish Keto, omit the carrots and potatoes, and reduce cook time to 2 minutes for soft cabbage, or just shut the lid and allow it to cook in the residual heat for firm cabbage.
Don't try to do this all in one step, or your veggies will turn to mush.