Grease a dutch oven or a round 2 quart oven-safe bowl. You can use cooking spray or butter.
In a large bowl, combine the whole wheat flour, oats, brown sugar, baking soda, baking powder and salt using a whisk.
In another bowl, combine wet ingredients including melted butter, vanilla, buttermilk and Guinness beer. Whisk together to combine.
Add the wet ingredients to the dry ingredients and stir until all mixed together and dough feels like a batter.
Next take the beer mixture and pour it into the dry ingredients. Gently stir until you get a well blended batter. This will not look like a typical bread dough, but more of a batter.
Pour batter into greased baking dish. Sprinkle with additional oats.
Bake at 425 degrees for 30 minutes. Then reduce the heat to 400 and bake another 30 minutes. Lastly, turn the oven off, crack the oven door open, and keep the Irish Guinness brown bread inside to cool for 30 minutes. Remove and enjoy!
If you'd like to substitute something for the Guinness stout beer you can try Ginger Ale, however it won't taste exactly the same.