Set Instant Pot to Sauté function and add oil. Once hot, add beef, searing on all sides until browned, about 4 to 5 minutes. Remove beef from insert.
Add onion, celery, and carrots, cooking until the onions are softened, about 5 minutes. Stir in garlic and cook for 30 seconds. Stir in tomato paste and cook for another minute. Add Guinness and cook for another 5 minutes.
Stir in broth, potatoes, worcestershire, bay leaf, thyme, red pepper flakes, and beef. Set Instant Pot to Manual function. Cook for 20 minutes on High pressure. Release pressure manually.
In a small bowl, whisk together cornstarch and water to make a slurry. Stir into the stew, letting the stew cook on warm for a few minutes until slightly thickened.
Serve over mashed potatoes with a garnish of parsley.