Gather ingredients and tools.
Prepare the meat by removing it from packaging, saving the spice packet. Run under water to rinse quickly. Depending on the size of your Instant Pot and your corned beef, you may want to cut the meat into 2-3 chunks. For my 3.6 lb. roast, I cut it into two pieces since I want to be able to slice it later.
Put the meat in the bottom of your inner pot. I like to put the meat with the fat side down, if possible.
Quarter the onion and remove the skin. Chop the celery into a few large chunks as well. Peel the garlic cloves by smashing them with a knife on the cutting board to loosen the hard skin.
Add the vegetables to the pot with the meat, and sprinkle the spice pack into the pot. Drop in the bay leaves.
Pour the Guinness over top of everything.
Put the lid on the Instant Pot, close the vent, and set it to cook on manual, high, for 90 minutes. When the time is up, let it do a natural pressure release for 10 minutes.
Using a slotted spoon, carefully lift the corned beef out of the liquid and set it to rest on the cutting board. Strain the liquid and reserve.Note: the meat will be very tender and smell delicious, but refrain from slicing it until it has a few minutes to rest. Cutting before the meat has rested will cause the juices to run out and will give you a dry corned beef.
After 5-10 minutes resting, you can cut a few slices off. Try to only slice what you need for your meal. The meat is very soft and will tend to shred.
Put the rest of your corned beef into an airtight container and pour back in some of the juices from the pot. This will help keep it moist. Let it rest in the refrigerator overnight.
Once cooled and rested, you can use a sharp knife to carefully cut the corned beef into thin slices.
When reheated in a pan or in the microwave with some pot juices, the highly-marbled wagyu corned beef will literally melt apart.
Enjoy your sliced corned beef in sandwiches, tacos, or however you like! If you want to do corned beef and cabbage, you can cook the cabbage (and potatoes, carrots, etc.) separately, using the juices from the pot.