Mix the whole wheat flour thoroughly with the white flour. Rub the butter into the flours. Add the salt, and soda.
Make a well in the center and gradually mix in the liquid. Stir with a wooden spoon. You may need less, or more liquid - it depends on the absorbent quality of the flour. the dough should be soft but manageable.
Knead the dough into a ball in the mixing bowl with your floured hands.
Put in on a lightly floured baking sheet and with the palm of your hand flatten out in a circle 1 1/2 inches thick. with a knife dipped in flour, make a cross through the center of the bread so that it will easily break into quarters when it is baked.
Bake at 425 degrees for 25 minutes, reduce the heat to 350 degrees and bake a further 15 minutes.
If the crust seems too hard, wrap the baked bread in a damp tea cloth. Leave the loaf standing upright until it is cool.
The bread should not be cut until it has set - about 6 hours after it comes out of the oven.
From "The Art of Irish Cooking" by Monica Sheridan