Preheat the oven to 375°F.
In a large bowl, measure and stir together all the dry ingredients – the flours, brown sugar, baking powder, baking soda and salt. Cut in the butter (or pulse it in a food processor) until well blended.
In a medium bowl, stir together the buttermilk, egg and orange zest, and add it to the dry ingredients. Stir with a spatula until it’s about halfway mixed, then add the currants and stir just until combined.
Sprinkle the countertop with flour or oats and gently knead the dough, folding it over itself about 10 times, then either shape it into a ball (for a single Irish soda loaf) or divide it into balls about the size of a plum or small orange. Arrange about an inch apart on a rimmed baking sheet that has been sprayed with nonstick spray.
Brush the top (or tops) with milk and sprinkle with coarse sugar; cut an ‘X’ lightly on the top using a sharp serrated knife. Bake for 45-55 minutes for a large loaf, 30 minutes for small buns, until golden. Transfer to a wire rack to cool completely.