On medium-high heat, add the onion halves, flat side down and cook until lightly brown, 2-3 minutes. Remove and set aside.
Cook the carrots for the same amount of time as the onion and set aside.
Last, cook the garlic cloves for about 1 minute and set aside.
If there is any leftover fat/oil, carefully drain it from the pot.
On high heat, add the beer and scrape with a rubber spatula to loosen up the brown bits, and stir. Turn the heat down to medium-high.
Add the tomato paste and Worcestershire sauce and stir (or whisk) until combined. Add the onion, lamb and carrots; add the beef stock, celery stalks and the tied herbs.
Bring the stew to a boil, covered, and them turn the heat down to a low simmer. Simmer for about 3 hours.
OPTIONAL: If you are going to add the barley, bring 1-1/2 cups water or chicken stock (or a combination of both) to a boil. Add the barley, cover and cook on medium-low until the liquid has evaporated--20-30 minutes. Set aside.
Final Touches: Taste the liquid for seasoning. I make my own beef stock, and I don't salt it. Add salt, to your liking.
To thicken the stew: Whisk 4 Tablespoons cornstarch and cold water until smooth and lump-free. Very slowly, while stirring, pour HALF the cornstarch mixture into the stew. Wait a minute or two, for the sauce to thicken. Add the barley, and wait a few more minutes to see how much it thickens. Only add the remaining cornstarch mixture if the stew hasn't quite thickened enough so that Homemade Irish Soda Bread can sop up that tasty gravy.