Make the pastry:

To make the pastry in a food processor:

• Add the flour and salt and butter. Pulse until you get the texture of fine breadcrumbs. With the processor running, slowly drizzle in cold water until it forms a ball. You may need more or less water depending on the dough.

To make the pastry by hand:

• Add the flour, salt and butter to a large bowl. Using your fingertips (or a pastry cutter) rub the butter and flour until it resembles breadcrumbs. Drizzle in the water and mix using a fork until the it starts to come together and holds together when pressed in your hand. You may not need all of the water. Remove and shape into a ball on a floured board. Wrap in plastic wrap and refrigerate for 30 minutes or overnight.

Make the filling:

• If using leftover corned beef, blend the corned beef in a food processor, or chop very finely (if using canned/tinned skip this step). Set aside.
• Add the oil and butter to a pan over medium heat. When bubbly add the onions and a little salt, and sweat the onions until soft, don’t brown. Set aside.
• To a mixing bowl, add the corned beef, mash and onions. Use a fork to mix the mash into the corned beef until it resembles a paste-like consistency. If you are using leftover corned beef and it seems dry, you can add a little water to make it more paste like.
• Remove the dough from the refrigerator and cut in half. Wrap one half back in the plastic wrap and refrigerate.
• Preheat oven to 400°F/200°C.
  Liberally grease a 9-inch (23 cm) pie dish with butter.
• Roll out the dough on a floured surface, a little larger than your pie pan, trim the edge so it’s even. If the dough is too firm, allow to come to room temperature so the butter softens. Roll the dough over your rolling pin and place into dish.
• Cut a large circle of parchment paper or foil larger than the pan or and place on top of the pastry in the pie dish. Pour the dried beans to the center to weigh down the dough and stop it from rising. Or, place a piece of foil on top of the dough and weigh down with an oven-proof dish that will fit inside. Bake for 12 minutes until you start to see the edges get a little golden brown. Remove the crust from the oven, grab the corners of the paper or foil and remove the beans If it puffs, it should go down. Keep the beans, you can reuse them over and over.
• Return the crust to the oven for 5 more minutes to cook. Remove from the oven and allow to cool slightly. This is called blind baking which partially cooks the bottom crust, you can skip this step.
• Fill the pan with the corned beef filling and smooth the top. Brush the edge of the crust with the egg wash.
• Roll out the other half of the dough and roll over your rolling pin, place on top of the pie. Press the edges to seal and trim the overhanging edge of the pastry. Brush the top with egg wash. Cut a shamrock from the trimmed pastry (optional), brush with egg wash.
• Bake for 35-40 minutes until the crust is golden brown, time will depend on your oven. Allow to rest for 10 minutes before slicing.