Season the beef cubes with salt and pepper. Heat a large skillet over medium-high heat and brown the beef in batches until all sides are seared. Transfer the beef to the slow cooker.
Add the diced onion and minced garlic to the same skillet and cook for 2 minutes until softened. Stir in the flour and tomato paste, then pour in the Guinness to scrape up any browned bits.
Transfer the onion mixture to the slow cooker. Add the potatoes, cabbage, carrots, beef broth, thyme, and bay leaves.
Stir everything to combine, cover, and cook on low for 8 hours until the beef is fork-tender.
Remove the bay leaves, taste and adjust salt and pepper if needed, then stir in the chopped parsley just before serving.