Rinse brisket and pat dry. Season top of brisket (fat side up) with pepper.
Place the roasting rack into the inner pot of a 6 or 8 quart pressure cooker. Pour in Guinness and beef broth. Add the contents of the spice packet, smashed garlic cloves and bay leaves.
Place the seasoned brisket (fat side up) on top of rack.
Place the lid on pressure cooker and lock. Steam release knob should be set on "sealing". Cook on manual setting (high pressure) for 95 minutes. When finished cooking, naturally release the pressure for 15 minutes. Quick release the remaining pressure.
Once the pressure is completely released, remove lid, carefully remove the brisket (using the handles on the rack) and place the brisket on a large serving platter. Cover with aluminum foil. Do not discard the liquid in the pot.
Add potatoes, carrots, cabbage and onion to the liquid in the pot. Ensure that you do not go over the fill line. Place lid on pressure cooker and lock. Steam release knob should be on "sealing". Cook on manual setting (high pressure) for 4 minutes (note that it will take longer to come to pressure than when you were cooking the brisket).
When vegetables are finished cooking, quick release the pressure. Once pressure is completely released, carefully remove lid. Using a large slotted spoon, transfer vegetables onto platter with the brisket. Remove and discard bay leaves. Thinly slice brisket against the grain. Ladle desired amount of liquid over brisket and vegetables.
Sprinkle with chopped fresh parsley when serving, if desired.