Step 1: Prepare the Pickle-Brined Cabbage (Optional, but Highly Recommended). In a small bowl, combine the dill pickle brine and water. Take half of your chopped cabbage and add it to the brine, making sure it’s submerged. Let it sit at room temperature for at least 30 minutes, or refrigerate for up to a few hours.

Step 2: Braise the Corned Beef. Put the corned beef brisket in a large pot or Dutch oven with the thickly sliced onion and celery pieces. Sprinkle the contents of the spice packet over the top. Pour in enough cold water to cover the brisket.

Step 3: Bring to a Boil and Skim. Turn the heat to high and bring the water to a boil. Skim off and discard any foam that rises to the surface, if desired.

Step 4: Simmer. Once boiling, reduce the heat to low, cover the pot, and simmer gently until the corned beef is tender, about 3 ½ hours. Use an instant-read thermometer to check for doneness – it should read at least 145 degrees F (63 degrees C).

Step 5: Cool the Corned Beef. Carefully remove the corned beef from the pot and transfer it to a bowl. Let it cool slightly. Taste the broth and add salt if needed.

Step 6: Cook the Cabbage. Bring the broth back to a boil over high heat. Add the remaining (non-brined) cabbage to the boiling broth. Cook until the cabbage just starts to soften and sweeten up, about 5 minutes.

Step 7: Remove Cabbage. Use a slotted spoon to remove the cooked cabbage from the pot and transfer it to a bowl.

Step 8: Cook the Carrots. Add the thickly sliced carrots to the boiling broth. Cook until the carrots start to soften, about 5 minutes.

Step 9: Remove Carrots. Use a slotted spoon to remove the cooked carrots from the pot and transfer them to a bowl.

Step 10: Cook the Potatoes. Add the quartered potatoes to the boiling broth. Reduce the heat to medium and cook until the potatoes are tender, 15 to 20 minutes.

Step 11: Slice the Corned Beef. While the potatoes are cooking, slice the cooled corned beef along the fatty seam to separate it into 2 pieces. Identify the direction of the meat fibers (the grain) and cut across the grain to slice the beef into ¼-inch thick slices. Trim off and discard any large pieces of fat.

Step 12: Preheat Oven. Preheat the oven to 400 degrees F (200 degrees C). Butter a 9×15-inch casserole dish.

Step 13: Mash the Potatoes. Use a slotted spoon to remove the cooked potatoes from the pot and transfer them to a bowl. Reserve the broth. Add the butter slices, salt, pepper, cayenne pepper (if using), and milk to the potatoes. Mix and mash until very smooth.

Step 14: Add Cheese to Potatoes. Toss in ¾ cup of the grated Irish Cheddar cheese and mix until incorporated.

Step 15: Assemble the Shepherd’s Pie. Pour the non-brined cooked cabbage into the prepared casserole dish and press on it until nicely compacted. Evenly distribute and press the cooked carrots over the cabbage. Place an even layer of the sliced corned beef on top, overlapping the slices slightly and distributing the fatty pieces evenly.

Step 16: Optional Relish Layer: If using, spread a thin layer of dill pickle relish evenly over the sliced corned beef.

Step 17: Add Brined Cabbage. Drain the pickle-brined cabbage and arrange it evenly over the relish layer (or directly over the corned beef if not using relish).

Step 18: Add Broth. Pour 1 cup of the reserved broth over the filling.

Step 19: Top with Mashed Potatoes. Dollop the mashed potatoes over the top of the filling. Spread the potatoes into a uniform layer with the back of a spatula.

Step 20: Create Texture. Switch to a fork and push the potatoes to the edge of the dish to create a seal. Then, drag the fork sideways back and forth over the potatoes to create a textured surface. Repeat this texture, going top to bottom over the potatoes.

Step 21: Sprinkle with Cheese. Sprinkle the top of the mashed potatoes with a little more grated Cheddar cheese.

Step 22: Bake. Bake in the center of the preheated oven until beautifully browned and piping hot, 45 minutes to 1 hour.

Step 23: Rest. Remove the Shepherd’s Pie from the oven and let it rest for 10 minutes before serving.

Step 24: Warm Remaining Broth. While the Shepherd’s Pie is resting, warm any remaining broth over medium-low heat.

Step 25: Serve and Garnish. Cut the Shepherd’s Pie into squares. Garnish with chopped scallions. Serve with the warmed broth. And, of course, serve with chilled dill pickle spears on the side!