Heat the olive oil in a skillet over medium-high heat. Add the beef cubes and brown on all sides, about 5 minutes total. Transfer the beef to the slow cooker.
Add the diced onion and minced garlic to the same skillet. Cook for 2 minutes until the onion softens, then scrape everything into the slow cooker.
Add the sliced carrots, sliced parsnips, cubed potatoes, Guinness, beef broth, tomato paste, thyme, and bay leaves to the slow cooker. Stir to combine.
Season with salt and black pepper. Cover and cook on low for 8 hours.
Remove the bay leaves. Ladle the soup into bowls and sprinkle with chopped parsley before serving.