Preheat oven to 425 degrees.
Whisk together the flours, salt, baking soda and seeds. Add the butter and rub it into the flour mixture with your fingers until it is blended. Make a well in the center.
Pour the buttermilk in the bowl with the flour. Using one hand formed into a claw, fold the flour and buttermilk together until the liquid is absorbed and it forms a soft dough.
Place the dough on a floured work surface and gently form into a round about 6 1/2 inches wide and 1 1/2 inches thick. Cut a deep cross on top and place on a baking sheet.
Bake for 15 minutes at 425 degrees, then turn down the oven to 400 degrees and bake for 30 minutes more. Cool on a wire rack and serve.
NOTE: Irish Soda Bread is best served the day it is made. I have refrigerated our leftovers and toasted it for several days after and it was still delicious.