From 1912:

Boil enough mealy Irish potatoes to make two cupfuls when they are done. In enough water to cover them deep. When the (peeled) potatoes are cooked and mealy, drain off the water and mix it to a batter with nearly a pint of flour. Mash and beat in the potatoes. The batter should be as stiff as you can stir with a spoon, as it will thin in cooling. When it is lukewarm stir in a yeast cake that has been softened in a little warm water. Beat well and set to rise in a warm place all night. In cold weather set it at noon for next morning. When the dough is light and covered with air bubbles, stir and beat hard, and set for the second rising (This will be in from four to six hours.)
In the morning put a good quart of buttermilk that is not too sour upon the stove to scald. Let it cool to lukewarmness. Sift about five quarts of flour into a big bowl, and stir in a tablespoonful of salt. Make a hollow in the center of the salted flour, pour in the milk and stir some of the flour down into it. Add the yeast sponge and knead all together steadily, working from the center toward the outer portions of the dough, and turning over and over to subject every part of the mass to the kneading, until the dough is smooth and elastic. Cover with a cloth and set to rise in a moderately warm place out of all drafts of cold air. When it is light and has doubled the original bulk, cut it down with a dull knife. Set for another rising; cut it down again when it is light, and set for the next to the last rising. The last rising is given when the risen dough has been made into loaves. Put it then into pans, and when they are light, bake.
This quantity will make four two pound loaves. If you have too much for one baking, put half of the sponge into a glass jar, cover it with cold water, and keep in the closed jar in a cool place for the next time.
It will keep good for three or four days in a cold place.