Fill a large pot with cold water, corned beef, onion, celery, Worcestershire sauce, thyme, and parsley. Put on high heat and bring to a boil. Once boiling, reduce heat to low, cover with a lid, and simmer until tender (about 2-3 hours). Once cooked thoroughly, remove corned beef. Taste the broth and add salt as needed.

Bring the broth back to a boil and add in the cabbage and carrots. Remove the cabbage and carrots once the carrots are fork tender.

Add the potatoes to the broth and cook until fork tender (about 15 minutes). While the potatoes are cooking, prepare the corned beef. Slice the beef along the fatty seam to separate into 2 pieces. Slice into ¼ inch thick slices against the grain. Trim and discard any big pieces of fat.

To make the mashed potatoes, use a slotted spoon to remove the potatoes from the broth. In a bowl, add potatoes, salt to taste, ground black pepper, cayenne, milk, and ¼ cup Irish cheddar cheese. Use a masher to mix until smooth.

Preheat the oven to 400°F. Grease a large cast-iron skillet or casserole pan. Pour cabbage and carrots into the skillet or pan and evenly distribute. Next, evenly layer the corned beef. Pour 1 cup of the broth over the top. Finish it off by spreading the mashed potatoes on top. Make sure to bring the mashed potatoes all the way up to the edge of the skillet or pan. Sprinkle remaining cheddar on top.

Bake for 45 minutes until golden on top. Let rest for 10 minutes before serving and garnish with scallions. Enjoy!