Heat 1 tbsp of olive oil and 1 tbsp of butter in a large dutch oven or other heavy bottom pot with lid over medium high heat until hot.
Generously season both sides of the beef stew meat with salt & pepper. In batches, so as not to overcrowd the beef, sear one side of the meat undisturbed for 1-2 minutes until browned. Flip meat to the other side and sear for an additional 1-2 minutes until browned. Using a slotted spoon or tongs, remove the seared beef (it will not be cooked through at this time) to a plate and set aside.
Repeat the process until all of the stew meat has been browned. This will probably take about 2-3 batches. Set aside.
Melt the remaining 1 tbsp of butter in the pan with the beef drippings and add in onion, celery, carrots and garlic. Sautee over medium high heat for about 5 minutes until softened.
Add in thyme, rosemary and tomato paste and cook for 1 minute over medium high heat.
Stream in the can of Guinness to deglaze the pan. Then pour in 6 cups of beef broth, stirring continuously.
Add in 3 tbsp of Worcestershire sauce, browned beef and one bay leaf and bring to a boil.
Turn heat down to low then cover and let simmer on low for about 1 - 1.5 hours.
After 1 to 1.5 hours of cooking, add in 1 cup of pearled barley. Continue cooking on low for another hour or so until the barley is cooked through and the beef is tender.
Remove from heat and taste to see if the stew needs any additional seasoning. Add any salt, pepper or Worcestershire sauce to preference.
If desired, garnish with thyme leaves and enjoy!