Pat the corned beef cubes dry with paper towels. If using flour, toss the beef in a bowl with the flour until lightly coated.
Heat the vegetable oil in a large Dutch oven or heavy-bottomed pot over medium-high heat. Add the beef in batches and sear until browned on all sides. Remove the beef and set aside.
Reduce heat to medium. Add the chopped onion to the pot and sauté for 5 minutes until softened. Add the minced garlic and cook for another minute until fragrant.
Pour in the stout beer (or 1 cup of broth) to deglaze the pot, scraping up any browned bits from the bottom with a wooden spoon.
Return the browned beef to the pot. Add the remaining beef broth and the pickling spice packet (or cheesecloth bundle). The liquid should just cover the meat; add a little water if necessary.
Bring to a boil, then reduce heat to low. Cover and simmer for 1.5 to 2 hours, or until the beef is starting to become tender.
Add the potato chunks and carrot slices to the pot. Cover and continue to simmer for 20 minutes.
Stir in the chopped cabbage. Cover and cook for an additional 15-20 minutes, or until the vegetables are tender but not mushy.
Remove the pot from heat. Discard the pickling spice bundle. Season with black pepper (taste before adding salt, as the corned beef is naturally salty). Garnish with fresh parsley before serving.