If you can get mutton, that's ideal for this dish, but if you can't, use lamb. You want it to have some fat on it, and on the bone is better - chops are ideal, or maybe shanks. Lean diced lamb isn't really the thing here - we need the fat and bones for flavour. If you decide to use beef, then it's not Irish stew (though it would still be very nice), and this is one thing that you definitely shouldn't put Guinness in - save it for drinking!
Peel and chop your onion into small bits. Heat some oil in a frying pan and sauté the onion until browned and caramelised a bit. Remove them from the pan, then add the chops, and get them browned all over. You may need to do them in batches if the pan is too crowded, and you may also need to add a bit more oil. When browned, put them in a large pot with the onions and the stock. Bring to the boil, then reduce to a simmer, put a lid on and cook for an hour.
Peel the potatoes and carrots and cut them into bite-sized chunks. Cut the celery into smaller chunks. Add the vegetables to the pot, along with the barley. You may need to add more water to make sure everything is covered. finely chop the parsley and throw that in too. Bring to the boil, then turn the heat down and simmer with the lid on for at least another hour. Taste for seasoning and add salt and ground black pepper to taste.
Serve with a good chunk of soda bread and butter.
Salt and black pepper to taste, oil for sautéeing