Heat a skillet over medium heat and brown the beef cubes in batches until they have color on all sides. Transfer the beef to the slow cooker.
Add the diced onion, carrots, celery, and minced garlic to the slow cooker. Sprinkle the flour over the meat and vegetables, then stir to coat.
Pour in the Guinness and beef broth. Add the thyme, bay leaves, salt, and pepper. Stir once, cover, and cook on low for 8 hours.
About 30 minutes before the stew finishes, place the potatoes and smashed garlic in a large pot. Cover with water and boil until the potatoes are fork-tender.
Drain the potatoes, return them to the pot, and mash with the butter and milk until smooth. Season with a pinch of salt if needed.
Remove the bay leaves from the stew before serving. Spoon the stew over portions of the garlic mashed potatoes.