Preheat the oven to 425°F or gas mark 7. If using a cast iron skillet place in the oven when cold to preheat with the oven.
If using a food processor blend the flour, baking soda, and salt together. Add the buttermilk and blend until dough has formed. If using a bowl mix the flour, salt, and baking soda until combined and then add the buttermilk until a dough is formed. The dough will still be a little sticky and that is ok.
Shape the dough into a ball about 8 inches, 21 cm, in diameter. Slice an x in the top of the dough and bake for 35 minutes and then turn the soda bread upside-down and bake for another 5 minutes.