To make tortillas, in a large bowl combine masa, shredded potatoes, 1 tablespoon scallions and salt. Add water and use your hands to form into a dough. Add water, 1 teaspoon at a time, if dough seems to dry. Keep a damp towel over dough while you make tortillas.
Heat a large cast iron skillet over medium high heat. Take off a golf ball-size piece of dough and roll into a ball. Place between 2 pieces of plastic or wax paper and press in a tortilla press. Remove dough from the plastic and cook in skillet until lightly toasted, about 2 minutes per side. Remove and place tortillas in a stack on a plate. Keep covered with a dry, clean towel. Repeat with remaining dough.
In a large bowl, mix to combine the green cabbage, red cabbage, carrots, 2 tablespoons scallions, salt and pepper. In a medium bowl, whisk to combine mustard, mayonnaise and horseradish. Pour 1/2 cup sauce over cabbage and mix to combine. Reserve remaining sauce to drizzle over tacos.
In a medium bowl, mash to combine warm cooked purple potatoes with olive oil, salt and pepper.
To assemble tacos, spoon purple potato mash into the center of a tortilla. Top with cabbage slaw and corned beef. Drizzle with horseradish mustard sauce and garnish with cilantro, chilis and a lime wedge.