Cook bacon in stew pot over medium high heat until browned. Remove and set aside on a large plate or in a large bowl.
Pat dry beef chunks and season with salt and pepper, if desired. Sear the meat in the stew pot, working on at least 2 sides of the meat, about 5 minutes total. Set aside the browned meat with the bacon.
Turn the heat down to medium, then add the onions, carrots, and parsnips to the stew pot, sautéing until softened, about 7 minutes. Sprinkle in the flour and cook another minute.
Pour in the Guinness, then scrape the bottom to remove any browned bits. Add in the garlic, celery, and tomato paste. Let cook for about a minute.
Return the beef and bacon to the pot, along with any juices from the plate or bowl. Break the bacon up into chunks as you go.
Add the thyme, bay leaves, and enough stock to just cover everything. Bring to a simmer and let cook, covered, for 2 hours, or until the beef is fork tender.
If it's still a little thin, remove the lid and increase the temperature to medium high. Cook about 15-20 minutes, or until the desired consistency is reached.
This stew is excellent served on top of mashed potatoes or with a nice loaf of bread.