In a mini-food processor, fitted with a metal blade, pulse the oatmeal until the meal is very fine. Remove from the processor and turn into a mixing bowl. Stir the buttermilk into the oats and cover with plastic wrap. Let the oatmeal steep in the buttermilk for 12 hours.
In a mixing bowl, sift the other ingredients together. Stir in the steeped oatmeal and mix well. If the mixture is too wet add a little more flour.
Preheat the oven to 350 degrees F. Grease a 9-inch round cake pan with the butter.
Lightly dust the work surface with some flour. Turn the dough out onto the surface and knead the dough a couple of times to form a soft ball like shape. Press the dough into a round about 2 inches thick. Using a sharp knife, make a criss-cross cut into the bread. Place the bread in the prepared pan. Bake until golden brown, about 25 to 30 minutes. Remove from the oven and cool on a wire rack. Serve the bread warm with butter.
Recipe adapted from Irish Country Cooking by Darina Allen, published by Viking Penguin, 1996