Heat the vegetable oil in a large Dutch oven over medium-high heat. Add the beef cubes in a single layer and brown on all sides. Remove the beef and set it aside.
Add the onion to the same pot and cook until it softens. Stir in the garlic and cook for one minute more.
Pour in the Guinness and scrape the bottom of the pot to loosen any browned bits. Let it simmer for two minutes.
Return the beef to the pot along with the beef stock, tomato paste, thyme, bay leaves, salt, and pepper. Bring everything to a boil, then reduce to a low simmer. Cover and cook for 90 minutes.
Add the carrots, parsnips, and potatoes. Continue simmering uncovered for 45 minutes until the vegetables are tender and the beef is very soft.
Remove the bay leaves. Taste and adjust salt and pepper if needed. Stir in the parsley just before serving.