1.Set Instant Pot to sauté on high. Season meat with salt, pepper, and garlic powder. Heat oil, then brown meat in 2-3 batches. Once browned, remove from pot and set aside on a plate.
2.Add onions, celery and garlic to pot and cook until softened, 4-5 minutes. Add tomato paste and cook for 1 more minute. Pour Guinness over vegetables, scrapping any browned bits off the bottom of the pot.
3.Simmer until beer has reduced by half, about 5 minutes. Then switch off instant pot and stir in beef broth, potatoes, carrots and browned meat. Close lid and set valve to "sealing". Press "stew" button, which is 35 minutes.
4.Once pot is indicating it's done with cooking, wait for it to naturally pressure release for 20 minutes. Then manually release any remaining pressure and open the lid.
5.Mix cornstarch with water. Set instant pot to sauté and stir in cornstarch slurry. Simmer until thickened. Taste for seasoning and add parsley. Stir one last time and serve.