Preheat your oven to 400°F. While it heats, grease a cast iron skillet or 9-inch round baking dish with 1 tablespoon of butter, making sure to coat the bottom and sides evenly. This prevents sticking and helps the biscuits develop a golden crust.
In a large bowl, whisk together the flour, salt, baking soda, sugar, and fennel seed. The fennel seed adds a subtle Irish character and improves the texture of the crumb. Mix thoroughly to ensure the leavening agents are evenly distributed throughout the flour.
Pour 1 1/2 cups of buttermilk into the dry mixture and stir gently with a wooden spoon until a shaggy dough forms. Don't overmix—this keeps the biscuits tender. If the dough seems too dry and won't come together, add a splash more buttermilk, but remember that Irish soda bread should be slightly sticky rather than smooth. Turn the dough onto a lightly floured counter and knead gently 4-5 times until it just comes together into a cohesive mass.
Flatten the dough from Step 3 into a 1-inch thick disk on your counter, then divide it into 5 or 6 equal portions. Roll each portion gently into a ball and arrange them in your prepared skillet. I like to space them close together so they support each other as they bake, creating slightly fluffier interiors while the edges get crispy.
Place the skillet in your preheated 400°F oven and bake for 15 minutes. The biscuits will begin to set and develop a light golden color on top. While they bake, melt the remaining 3 tablespoons of butter in a small saucepan over low heat or in the oven's residual heat.
Remove the skillet from the oven and brush the tops of each biscuit with the melted butter from Step 5. Return the skillet to the oven and bake for another 5-10 minutes until the biscuits are golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs.
Remove the biscuits from the oven and let them cool in the skillet for 5 minutes so they set slightly, then transfer to a wire rack to cool completely. These are best enjoyed warm or at room temperature with butter and jam, or alongside a hearty stew.