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1
Whisk flour, salt and sugar; cut in butter until mixture resembles small peas. Gradually add beer, tossing with a fork until dough holds together when pressed. Shape into a disk; wrap and refrigerate 1 hour or overnight.
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2
Meanwhile, place potatoes and salt in a small saucepan; add water to cover. Bring to a boil. Reduce heat; simmer, uncovered, until potatoes are crisp-tender, 6-8 minutes. Drain. Return to saucepan; add green onions, butter, mustard and pepper. Cook over medium heat, stirring frequently, until potatoes are tender and browned, 6-8 minutes. Remove from heat; cool.
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3
Roll dough between two pieces of waxed paper to a 1/8-in.-thick, 12-in.-diameter circle. Remove top piece of waxed paper; place a 9-in. pie plate upside down over crust. Lifting with waxed paper, carefully invert crust into pan. Remove waxed paper. Trim crust to 1/2 in. beyond rim of plate; flute edge. Refrigerate 30 minutes.
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4
Preheat oven to 400u00b0. Prick bottom and sides of crust with a fork; line with a double thickness of foil. Fill with pie weights, dried beans or uncooked rice. Bake on a lower oven rack until edges are light golden brown, 15-20 minutes. Remove foil and weights; bake until bottom is golden brown, 3-6 minutes longer. Cool slightly.
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5
To assemble the rustic pie, layer half the corned beef, half the cheese and three-fourths of potato mixture in baked crust. Repeat corned beef and cheese layers; sprinkle with remaining potato mixture. Reroll dough trimmings; use a heart-shaped cookie cutter to make shamrock petals and a knife to cut out a stem. Brush dough trimmings with milk. Bake until crust and cheese are golden brown, 35-40 minutes.
