The great thing about Irish Stew is that you can't go wrong. There is no exact formula to making a perfect stew as it is a peasant dish therefore is meant to fit what you have on hand. I do equal amounts of the carrots, cabbage and potatoes, half amount of onion and turnip which is optional and usually half the amount of the other vegetables. Look at what you have on hand and fit your stew to match that. For a small cabbage head, I use about 1.5-2lbs of meat. For the herbs I use a few teaspoons of each and at least 4 cloves of garlic. I brown the meat in the olive oil and Guinness in a cast iron pan and then add the red wine to pull off all of the goodness. Place meat in a crock pot or instant pot or large soup pot. Then add in vegetables and stock until you reach the desired amount of stock vs vegetables. Add in seasonings, a few teaspoons of each is a good starting place. Cook in crock for ~6 hours, or simmer in soup pot until meat in tender, or in an instant pot for 45 minutes. Adjust seasoning after soup is cooked.
