Combine onion, garlic, pickling spices, and water in the insert of a 6-quart Instant Pot. Place the corned beef brisket, fat side up, on a rack on top of the onions. Pour beer over the brisket.
Seal lid and set the unit to HIGH pressure for 85 minutes. Let pressure release naturally for 20 minutes, then manually release the remaining pressure.* Remove corned beef and place on a dish or a cutting board, spooning a bit of the cooking liquid over the top. Cover with foil to keep warm.
Strain cooking liquid and discard solids. Return 1-1/2 cups of liquid to the Instant Pot and add potatoes, carrots, and cabbage wedges. (Reserve remaining cooking liquid.) Seal lid and set the unit to HIGH pressure for 3-4 minutes, depending on how soft you like your vegetables. Quick release pressure.
Slice corned beef against the grain. Spoon a little bit of the reserved cooking liquid over the corned beef slices on your platter. Sprinkle parsley over the vegetables, and if desired, drizzle with melted butter. Serve corned beef with mustard.
*Since the beer foams up in the pressure cooker during the corned beef cooking process, you might find that the initial force of manually releasing the steam (after the 20 minute natural release) causes some liquid to spray out of the valve. Sometimes it's happened to me, other times, it hasn't. A little bit of liquid is fine, but if you release the pressure and find that too much liquid is coming out, re-seal the valve, and try releasing the remaining pressure again in another 5 minutes. Or, release the steam more slowly by turning the valve just until you hear a hissing noise and carefully holding it in that position (be sure to wear a heat-proof glove).Read more about gradually releasing the steam in this article, section #3.Some have recommended covering the lid/valve with a towel while releasing the pressure. Do not do this, as obstructing the vent can damage your pressure cooker and cause a safety issue.
To Glaze The Corned Beef (Optional)
This corned beef is also delicious served with my Irish Whiskey-Mustard glaze. The ingredients and directions can be found in my recipe, Beer Braised Corned Beef with Whiskey Mustard Glaze. After fully cooking the corned beef in the Instant Pot (instead of Dutch oven braising), follow the directions to "Glaze and Serve".