Preheat the oven to 425°F. Flour a 9- or 10-inch cast-iron skillet, or line a baking sheet with parchment paper.

Place the flour, sugar, baking soda, and salt in a large mixing bowl. Mix with a rubber spatula to combine.

Using the coarse holes of a box grater or hand grater, grate the butter directly into the bowl of flour. Add the currants or raisins to the bowl. Gently mix with a rubber spatula to distribute butter pieces and dried fruit.

Whisk together the buttermilk and egg in a medium bowl, then pour over the flour-butter mixture. Gently mix the wet ingredients into the dry with a rubber spatula, working until they combine in a tacky dough.

Turn dough onto a well-floured work surface. Shape into a round loaf, and use a sharp knife to score an X" into the top. Carefully transfer loaf to the center of the floured skillet.

Bake the loaf on middle rack of oven until it is golden brown and sounds hollow when tapped, 40-45 minutes.

Check to see that bread is done. Remove from oven or add time as needed.

Allow bread to cool for at least 1 hour before slicing and serving. Leftover Irish soda bread can be kept in an airtight container for up to 3 days, or frozen for 3 months.