Heat the oven to 375°F and grease a 9 inch by 5 inch loaf pan with butter or cooking spray.
In a large bowl, whisk together the whole wheat flour, all-purpose flour, dark brown sugar, baking soda, and salt.
2 cups (250 grams) whole wheat flour, 2 cups (250 grams) all purpose flour, ¼ cup (50 grams) dark brown sugar, 1 teaspoon baking soda, 1 teaspoon salt
Add the Guinness, buttermilk, and vegetable oil and mix only until all traces of flour have disappeared. It will be a rough, shaggy dough.
1 cup (220 milliliters) Guinness draught, 1 cup (240 milliliters) buttermilk, 2 tablespoons neutral oil
Gently pour the soda bread mixture into the prepared pan. Scatter the rolled oats over the top and press them gently so they adhere.
¼ cup rolled oats
Bake at 375°F for about 50 to 60 minutes, until the center of the loaf reaches 200°F on an instant-read thermometer. Remove the loaf from the pan to a cooling rack - it should give a slightly hollow sound when tapped gently on the bottom.