Remove corned beef from package and reserve seasoning packet (if there is one). Rinse corned beef under water and trim any excess fat. 

Add a small trivet to the base of the pressure cooker. Place 2 cups water to the bottom of the cooker along with the seasoning packet. Place corned beef (fatty side up) on the trivet. 

Remove onion peel and chop roughly in eighths. Wash celery stalk and roughly chop as well. Add vegetables to the top of pressure cooker and lock lid in place. 

Cook corned beef on high pressure for 50-65 minutes. The exact amount of time will depend on the size of corned beef (see note). When cooking time is up, release pressure naturally. 

Test meat tenderness with a fork. The corned beef should be tender enough for a fork to slide in an out easily. If it is not done, replace lid and bring back up to pressure for the 5 to 10 minutes (depending on how tender you want it).

While meat is cooking wash cabbage and cut in quarters. Remove the core and then cut each quarter in half. 

When meat is completely cooked and pressure is released, remove lid and carefully add cabbage to pressure cooker on top of the meat, making sure you still have plenty of room for the lid to lock properly in place. Replace lid and cook on LOW pressure for 1 minute. Then release pressure naturally.

When pressure has released, remove cabbage and corned beef to a serving platter and serve warm.