Dry the lamb and season generously with salt and pepper.
Add the lamb, half the potatoes, and onions to a very large Dutch oven or heavy soup pot. Pour in the beer and add enough water to just cover the ingredients. Bring to a simmer over medium-high heat, then lower the heat, partially cover the pot, and keep the stew at a gentle simmer for about 1 hour, stirring occasionally. Keep the pot partially covered. Add more water if the liquid seems to be evaporating too quickly; you want the stew to be saucy. Skim off any foam that accumulates on top of the stew.
After an hour, smush up some of the potatoes against the side of the pot to thicken the stew a bit. Add the rest of the potatoes, the parsnips, and the carrots. Return to a simmer. Continue to simmer for 30 more minutes, partially covered, stirring occasionally. Uncover the pot and continue to cook until the meat and all of the vegetables are tender, about 15 to 30 minutes more. Add more water if needed. If desired, to thicken the stew further, use the back of a spoon to crush some additional cooked potatoes against the side of the pot and stir to blend them back into the stew.
Taste and adjust the seasonings as needed. Stir in the cup of parsley. Serve hot in bowls garnished with additional chopped parsley.
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Notes
Guinness beer is a traditional choice for Irish Lamb Stew, but you can use any beer you like. Steer clear of beers that are very hoppy or sweet.
Use the back of your spoon to crush some of the potatoes against the side of the pot at the end and stir them into the rest of the stew. The rest of the potatoes and the other root vegetables will have had enough time in the pot to become very tender but still hold their shape.