Preheat oven to 375°F and line a baking sheet with parchment paper.
Whisk 4 ¼ cups of the flour, sugar, baking soda and salt together in a large bowl. Cut in the butter using a pastry cutter, a knife or your hands.
Use a measuring cup to measure out the buttermilk. Whisk in the egg and orange zest.
Pour the wet ingredients into the dry ingredients. Stir the dough with a rubber spatula and add in the remaining ¼ flour, a bit at a time, until the dough is not too sticky.
Tip the dough onto a floured surface. With floured hands, knead the dough a few times until it just stays together. Combine the raisins with 1 tablespoon of flour and then knead them into the dough.
Gently form the dough into a round and place it onto the prepared baking sheet. Use a very sharp knife and score an "x" onto the top of the loaf.
Bake until the loaf is golden brown and a toothpick inserted into the center comes out clean, about 45-50 minutes. The loaf will be 200°F when done. If the loaf is browning too much, place some aluminum foil on top.
The Irish soda bread will last up to 2 days at room temperature wrapped with aluminum foil. It will last for a week in the refrigerator.