Cut the beef into 1 ½ -2" chunks. Pat dry then sprinkle with salt and pepper.
Heat oil in a heavy-based wide pot over high heat (I used my All-Clad 6 qt 5 ply pot). Add beef and brown the pieces well on all sides. Remove and place on a plate until needed.
Lower the heat to medium. If needed add additional oil.
Add garlic, onions and mushrooms and cook for 3-4 minutes.
Add Bacon and continue to cook until the bacon is browned (about 5 minutes)
Add carrots and celery and continue to cook for 3-5 minutes.
Add flour to the pan and mix in well. Allow the mixture to cook for 2-3 minutes to get the raw taste out of the flour.
Add Guinness to deglaze the pan and get all the bits off the pan into the sauce. Then add the beef stock and tomato paste. Mix well.
Add the browned beef into the pot (including any juices) along with the potatoes and thyme.
Cover, lower heat so it is bubbling gently. Let the stew cook covered for 2 hours. At this point the beef should be pretty tender. Remove lid and let the stew continue to simmer for an additional 1-2 hours. The sauce will have reduced and be amazingly flavorful.
Skim off any fat that may be on the surface of the stew. Adjust seasonings to taste with sea salt and pepper. Remove thyme and serve.