Corned beef is beef brisket salt cured, traditionally for preservation. You can find corned beef in the grocery store already cured with a packet of spices. A traditional St. Patrick's Day meal in the US is Corned Beef and Cabbage.
In your instapot, placed sliced onions in the bottom. Place trivet over the onions.; This makes removal of the cooked beef easier. Place corned beef, fat side down on trivet. Sprinkle seasoning packet on the beef. Pour beer and water in the bottom of the instapot being careful not to wash the seasonings off the meat.
Put the lid on the instapot and set it on high pressure for 90 minutes. It will take about 20 minutes to come to pressure. When the 90 minutes is over, allow natural release for 20 minutes and then quick release the rest of the way.
Trim the fat off of the meat then cut in slices against the grain. Serve with steamed cabbage and creamy mashed potatoes.