Pat the beef dry. Season with half the salt and pepper.
Toss the beef with the flour until lightly coated.
Heat oil in a heavy pot over medium-high heat.
Brown beef in batches until deeply seared. Transfer to a plate.
Add onion, carrots, and celery. Cook until softened, about 6 minutes.
Stir in garlic and tomato paste. Cook one minute to toast.
Pour in Guinness. Scrape up browned bits from the pot bottom.
Add broth, Worcestershire, thyme, and remaining salt and pepper.
Return beef and juices. Bring to a simmer and cover.
Simmer gently until beef is nearly tender, about 90 minutes.
Stir in potatoes. Cover and simmer until tender, 30–40 minutes.
Uncover and simmer 5–10 minutes to thicken as desired.
Taste and adjust seasoning. Finish with parsley and serve warm.