Position a rack in the center of the oven and preheat the oven to 450 degrees. Lightly flour a large rimmed baking sheet.
Sift all of the dry ingredients into a large, wide mixing bowl. Make a well in the center and pour in 1-1/2 cups of the buttermilk. Stir with one hand, fingers apart, moving in circles to incorporate the buttermilk into the dry ingredients. If necessary, add more buttermilk 1 Tbs. at a time until the dough just barely comes together. (The absorption rate varies depending on the brand of flour.) The dough should be soft—don’t overwork it.
Turn the dough out onto a well-floured surface and pat into a round about 6-3/4 inches in diameter and 1-1/2 inches high in the center. Invert the round so the floured side is on top. With a thin, sharp knife, score a cross on the dough about 1/4 inch deep and extending fully from one side to the other.
Transfer the dough to the prepared baking sheet and bake for 15 minutes. Lower the oven temperature to 400°F and bake until the bread is browned and sounds hollow when tapped on the bottom, another 20 to 30 minutes.
Cool to room temperature on a rack, about 2 hours, before slicing and serving.
*Though it’s not traditionally Irish, you can add 3 oz. raisins with the dry ingredients, or experiment with add-ins like freshly chopped herbs, olives, sun-dried tomatoes, caramelized onions, or chocolate chips.