Preheat your oven to 375°F. Line a large baking sheet with parchment paper or a silicone mat.
Whisk the 4 cups + 2 tablespoons all-purpose flour, 1 ½ teaspoon baking soda, and 1 ½ teaspoon salt together in a large bowl.
Add 6 tablespoons butter, chopped into small cubes, to the dry ingredients. Using either your hands or a bench scraper, work the butter into the flour mixture until it becomes crumbly.Make sure the butter is fully incorporated into the flour, with no large lumps remaining—each piece should be about the size of a pea. In a separate bowl, whisk 1 cup sourdough discard, 1 whole egg, ⅓ cup sugar, and 1 cup buttermilk until fully combined.
Add the wet ingredients and 1 cup raisins to the dry ingredients and mix using a baking spatula until it's just combined. Do not over-mix; it will be thick and sticky.
Turn the dough out onto a well-floured work surface. Form the dough into a round loaf. Score a deep "X" on the top of the loaf. Place onto the lined baking sheet.
Bake for 40 minutes, tent the loaf with foil, and bake for an additional 20 minutes. (The total bake time is 60 minutes.) The foil will help prevent the loaf from over-browning as it bakes. The bread is done when the internal temperature of the loaf is over 200°F (93°C).
Transfer the loaf to a wire rack to cool completely. Cut into wedges and serve. Irish soda bread is best enjoyed the day it's made.