Preheat oven to 375 degrees. Butter and flour a 10-inch cast-iron skillet; set aside.
In a large bowl, using your hands, mix together flour, sugar, baking powder, and salt. Add egg and mix to combine. Add raisins and stir until incorporated. Slowly add milk, mixing until a dough forms. If dough seems too stiff, add a little more milk until dough is the consistency of cookie dough.
Transfer dough to prepared skillet pressing evenly to fill. Transfer to oven and bake for 30 minutes. Decrease oven temperature to 300 degrees and continue cooking until top is golden brown, about 30 minutes more.
Remove soda bread from oven and let cool in pan for 10 minutes. Remove from pan and transfer to a wire rack to cool completely.