Add the olive oil to a large pot or Dutch oven and set it over medium-high heat. Sear the stew meat (all at once or in batches) until browned on each side. This will take approximately 8-10 minutes. Use a large slotted spoon to remove the beef to a clean plate and set aside. Save the leftover cooking juices in a small bowl or mug, as we'll be adding them back to the stew later on.
In the same pot or Dutch oven, cook the bacon over medium heat until cooked and crispy around the edges. Strain it from the grease and set it aside, leaving as much of the grease in the pan as possible.
Set the pot back over medium heat and add the chopped celery, carrots, and onion. Cook until the onions have softened, about 5 minutes or so.
Once soft, add the tomato paste, minced garlic, black pepper, and salt. Mix well to coat the vegetables in the tomato paste and cook, stirring often, for approximately 3 minutes. Add the fresh thyme and bay leaves.
Add the beef broth (reserving ¼ cup for the cornstarch slurry), Guinness beer, reserved cooking liquid, and seared stew meat. Scrape up any brown bits that may have stuck to the bottom of the pot.
Mix well and bring the stew to a low boil then reduce the heat to low, cover, and simmer for 1 hour.
Remove the lid and add the potatoes. Simmer for an additional 30-45 minutes, or until the potatoes are fork-tender.
Meanwhile, combine the cornstarch with the remaining ¼ cup of beef broth until completely combined. Add it to the stew and mix well to combine. Allow the stew to simmer for an additional 2-3 minutes more, or until thickened to your lining.
Finally, add the bacon back to the pot. Season with additional salt and black pepper, to taste. Serve with Irish soda bread and garnish with fresh thyme, if desired.