Place the beef and 3 tablespoons of flour in a medium bowl and toss so the beef is evenly coated in the flour.
Place 3 tablespoons of butter in the Instant Pot and set it on sauté for about 8 minutes. Once the butter is melted, add the beef. Let beef brown for at least a couple of minutes on one side and then stir to get a nice brown on all sides.
Add onion, garlic, carrots and celery to the Instant Pot and stir to combine. Pour in the Guinness to deglaze the pan.
Add potatoes, Worcestershire, tomato paste, beef broth, thyme and bay leaves to the Instant Pot. Stir to combine.
Turn the valve to Sealing, press the Meat/Stew button and select high heat for 35 minutes. The pot will take a bit of time to preheat, then it will begin to count down.
Once the time is up, turn the valve to vent. It will take a few minutes for all of the pressure to release. While you’re waiting, place remaining 2 tablespoons of butter in a small, microwave-safe dish and microwave until melted, about 30 seconds to 1 minute. Once butter is melted, stir in remaining 2 tablespoons of flour. Add this mixture to the stew in the Instant Pot and stir to combine. This will help thicken it up.
Remove bay leaves and serve warm.