Sprinkle your beef with salt & pepper and dredge each piece in flour.
Heat the oil in your Instant Pot on the "sauté" function. When hot, add the beef. I do 2 batches and brown each batch for about 5 minutes total (I use tongs to ensure all sides of the beef get browned). You may need to add more oil in the second batch.
Once the second batch of beef has browned, take it out of the Instant Pot. Add the onions in and sauté them for 2-3 minutes.
Add the Guinness, Worcestershire sauce, and garlic powder to the Instant Pot. Using a wooden spoon, scrape up any brown bits into the sauce.
Add the meat back in, along with the potatoes, celery, and carrots. Close the lid and make sure the valve is on "sealing". Cook on high pressure for 35 minutes.
You can let the pressure naturally release once the countdown has finished, but you can also do a quick release if you're in a hurry (I did).
Season stew with extra salt & pepper if needed. Serve immediately. It also freezes well and makes great leftovers too.