Preheat oven to 400 degrees. If using a cast iron skillet, place it in the oven at this time to start warming it up.
In at least a 2 cup liquid measuring cup, combine the milk and vinegar and allow to sit for 10 minutes. Start off by adding the 2 tablespoons of vinegar and then add milk until the volume of the liquid mix is 2 cups. The milk and vinegar combined shouldn't be more than 2 cups total.
In a large mixing bowl, add flour and whisk in the baking soda and salt.
Slowly add in the milk mixture with a fork or danish dough whisk. Make sure to whisk ingredients together as you combine them.
Mix until everything is fully combined and starts to form a ball. Soda bread is a very sticky dough so make sure your hands and work surface are lightly floured.
On a floured surface, remove dough from the bowl and using your hands, form the dough into a round loaf.
Remove skillet from the oven and add in 1 tablespoon of butter. This helps the bread from sticking and gives it a nice crispy crust.
Add dough to the skillet and with a serrated knife cut a deep cross in the top of the bread. about ½-3/4" deep.
Return to the oven and bake for 40 minutes, or until the top is golden brown. If the bread starts getting too brown you can cover with foil to prevent it from burning.
Remove from oven and place bread on a wire rack and allow to cool.
Slice, then spread on some Irish butter and serve.