1. Cook 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 of the corned beef.

Pour in enough cold water to cover the brisket completely.

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. This helps to create a clearer broth.

Once boiling, reduce the heat to low, cover the pot, and simmer gently until the meat is tender, about 3 ½ hours. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C).

2. Prepare the Vegetables and Broth:

Carefully remove the corned beef from the pot and transfer it to a bowl. Let it cool slightly.

Taste the broth and add kosher salt if needed.

Bring the broth to a boil over high heat.

Add the cabbage to the boiling broth. Cook until the cabbage just starts to soften and sweeten up, about 5 minutes.

Use a slotted spoon to remove the cabbage from the broth and transfer it to a bowl.

Add the carrots to the boiling broth. Cook until the carrots start to soften, about 5 minutes.

Use a slotted spoon to remove the carrots from the broth and transfer them to a bowl.

Add the quartered potatoes to the boiling broth. Reduce the heat to medium. Cook until the potatoes are tender, 15 to 20 minutes.

3. Slice the Corned Beef: While the potatoes are cooking, slice the cooled corned beef.

Slice the corned beef along the fatty seam to separate it into 2 pieces.

Identify the direction of the meat fibers (the grain). Cut across the grain (against the grain) to slice the corned beef into ¼-inch thick slices. This is crucial for tenderness.

Trim off and discard any large pieces of fat. Set the sliced corned beef aside.

4. Preheat the Oven:

Preheat the oven to 400 degrees F (200 degrees C).

Butter a 9×15-inch casserole dish (or a 9×13 if that is all you have.

5. Mash the Potatoes:

Use a slotted spoon to remove the cooked potatoes from the broth to a bowl. Reserve the broth.

Add the sliced butter, salt, black pepper, cayenne pepper, and milk to the potatoes.

Mix and mash the potatoes until very smooth. You can use a potato masher, a ricer, or an electric mixer.

Toss in ¾ cup of the grated Irish Cheddar cheese and mix until incorporated.

6. Assemble the Shepherd’s Pie:

Pour the cooked cabbage into the prepared casserole dish. Press on it until nicely compacted.

Evenly distribute and press the cooked carrots over the cabbage.

Place a nice, even layer of the sliced corned beef on top of the carrots, overlapping the slices slightly and distributing the fatty pieces evenly until the carrots are fully covered.

Pour 1 cup of the reserved cooking broth over the corned beef.

Dollop the mashed potatoes over the top of the corned beef. Then spread the potatoes into a uniform layer with the back of a spatula.

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 texturing process, going top to bottom over the potatoes.

Sprinkle with a little more shredded Cheddar cheese.

7. Bake:

Bake in the center of the preheated oven until the topping is beautifully browned and the filling is piping hot, 45 minutes to 1 hour.

8. Rest and Serve:

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

Meanwhile, warm any remaining broth over medium-low heat.

9. Garnish and Enjoy: Cut the Shepherd’s Pie into squares.

Garnish with chopped scallions.

Serve with the warmed broth on the side or drizzled over the top. Enjoy!