Recipe from America’s Test Kitchen:
2 cups (8 oz) whole wheat flour
1 cup (4.25 oz.) unbleached all purpose flour
1 cup of wheat bran
1/4 cup wheat germ
2 teaspoons of sugar
1 1/2 teaspoons baking soda
1 1/2 teaspoons baking powder
1 teaspoon table salt
2 cups of buttermilk
375° middle rack, 40-45 minutes, rotate 1/2 way through. 185° internal temp. Cool for 1 hr.
I found, on initial mix, the dough was far, far, too dry. It kept crumbling apart and WOULD NOT hold together.
I kept adding splashes of buttermilk until I could get a loaf that would hang together.
Baking time had to go a little longer too, after 45 minutes it was only 145°. Maybe because of the added buttermilk. Running another 15 minutes brought it up to temp.
As a daily consumer of brown soda bread in Ireland I feel obliged to say that recipe is not the best. Never ever have I seen sugar (it’s just not really a thing in bread here) and with buttermilk there’s no need for baking powder, only baking soda.
The Odlums recipe linked by another poster here is solid. Generally wouldn’t bother with wheatgerm or bran myself unless I had it to hand and I generally don’t though I have added it occasionally in the past. Decent option if you’re looking for extra fibre though.
Edit: on buttermilk I would generally add until it’s semi sloppy. Kinda do it by feel at this stage but I do recall that Odlums recipe being a good start before. You may need more if you’re adding in the bran / germ as I think they soak up quite a bit.
The trick with American “whole wheat” flour is that it’s nowhere near the same as Irish wheat flour. Adding the bran and germ is to make up for that missing consistency. We’re essentially having to re-create flour made with the whole grain.
That may be the cause for the sugar as well.