Trim the fat off the corned beef. Then cut the beef into 1-inch chunks. Chop all the vegetables.
Place the butter (or oil) in the base of an electric pressure cooker and set on Sauté. Add the chopped onions, and soften for 2-3 minutes. Then add the corned beef chunks and the included seasoning packet to the pot. Brown the meat for 3-5 minutes.
Add the potatoes, carrots, cabbage, thyme, and water to the pot. Lock the lid into place, and set on Pressure Cook High for 30 minutes. Meanwhile, shred the Dubliner cheese and toss it with the cornstarch, and set aside.
Turn off the pressure cooker, and perform a Quick Release. Once the steam valve button drops, you can safely remove the lid. If you didn't trim the fat well enough, there could be a thin layer of fat on the top of the soup. If needed, skim off the fat with a large spoon.
Turn the pressure cooker back on Sauté and stir in the cheese. Once the cheese has melted and the broth thickens, taste, then salt and pepper as needed.