By Mary O'Callaghan. She makes this at: Ballinalacken Castle Country House & Restaurant in Ireland. I've listed the times given in the recipe but please watch and use your own best judgement as they seem over long to me.
Whisk both flours, sugar, and baking soda and either option you might be using in medium bowl to blend.
Add margarine and cut in until margarine is reduced to pea-size pieces and add buttermilk; stir until shaggy dough forms.
Turn dough out onto lightly floured work surface and knead until dough comes together, about 10 turns.
Shape dough into 7 to 9-inch round (the round should be about 1 inch high).
Place dough on prepared baking sheet.
Cut large X, 1/2 inch deep, in top of dough.
Bake bread in center of oven for 15 min at 425 F then turn it down to 375 F for 40-45 minutes until deep brown and bottom sounds hollow when firmly tapped (a bamboo skewer inserted into the center of the bread should emerge clean without any stickiness or moistness), about 55-60 minutes total oven time.
Transfer bread to rack and cool completely.
Enjoy it's dense, hearty texture.
For this kind of dough, the less you handle and work it the better.