Wash and place potatoes at bottom of your Instant Pot or pressure cooker to create a trivet that the brisket will cook on.
Add onion and garlic. Place the corned beef brisket, fat side up, on top of the potatoes. Pour beer over the brisket. Top with pickling spice (or spices that came with the corned beef).
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.
Add carrots and cabbage wedges. 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. If needed, spoon a little more of the 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.