Cook bacon in a 6 quart Dutch oven over medium-high heat for 2-3 minutes or until crisp. Remove from pot using a slotted spoon.
Season beef cubes with 1 teaspoon salt and 1 teaspoon pepper on all sides. Add to bacon drippings in Dutch oven, browning on medium-high heat turning as need to brown on all sides. Cook in batches if needed, to avoid overcrowding the pot. Remove to a platter.
To the drippings add onion. Cook for 4-6 minutes or until softened and lightly browned. Add garlic, cook for 1 minute longer. Add butter stirring until butter has melted, scraping any brown bits from the bottom of the pot. Add flour, cook for 1-2 minutes until flour is absorbed.
Return beef to Dutch oven along with reserved ½ teaspoon salt, Guinness, beef stock, Worcestershire sauce, tomato paste. sugar, bay leaves and thyme sprigs. Bring to a boil, then immediately lower the heat and simmer covered for 1 hour.
Uncover and add carrots and celery to pot. Replace cover and continue to cook for an additional 1 hour. (To equal 2 hours total at this point) After simmering, test doneness of beef with a fork, it should be tender and shred easily. If not, stir well then cover and continue to simmer another 15-20 minutes, then test again.(If adding potatoes, do so at this point)
Once beef is fork tender, increase heat to medium-high and let bubble uncovered to reduce slightly for about 5 minutes. Taste and adjust salt and pepper to taste.
Remove bay leaves and thyme. Mix in cooked bacon and chopped parsley.
Serve ladled over mashed potatoes with soda bread and garnished with additional parsley or thyme.