Toss beef cubes with 2 tablespoons vegetable oil in a bowl.
Whisk flour, salt, black pepper, and cayenne pepper in a separate bowl. Dredge beef cubes through flour mixture until evenly coated.
Heat 1/4 cup vegetable oil in a large skillet over medium-high heat; cook and stir coated beef cubes, working in batches, until beef is browned on all sides, 5 to 10 minutes. Transfer browned beef to a Dutch oven or heavy-bottomed pot.
Cook and stir onions and garlic in the same skillet used for browning beef until lightly browned, 5 to 10 minutes.
Stir tomato paste with enough water to partially dilute; pour into onion mixture. Stir to blend. Reduce heat to medium, cover Dutch oven, and simmer for 5 minutes.
Pour half the Irish stout into the onion mixture, and bring to a boil while scraping the browned bits of food off of the bottom of the pan with a wooden spoon. Transfer entire mixture to the browned beef. Pour remaining half of Irish stout into beef mixture; add thyme.
Cover Dutch oven, reduce heat to low, and simmer for 2 hours. Add potatoes and carrots and simmer, stirring every 20 minutes, until potatoes are soft, about 1 hour. Adjust salt as needed; garnish with parsley.