For many peoples of the world, a staple of the local diet is a grain-based, breadlike product served at every meal. Across the continents of Central and South America, for example, this mealtime staple is the tortilla; in India, you'll find the chapati; and on the Emerald Isle of Eire, it's Irish Soda Bread.
Over much of their famine-plagued history, the Irish have struggled to keep the larder stocked and put the next meal on the table. Soda bread was the answer to keeping famine at bay, since it is made from dirt-cheap, readily available ingredients. It also became a cornerstone of traditional Irish cuisine, and recipes for soda bread are still zealously passed down from generation to generation.
In the industrialized West, modern bread typically employs yeast as a leavening agent to cause the dough to rise as it bakes. The leavening process produces bubbles of gas (carbon dioxide), which gives the bread a softer, lighter texture, and makes it considerably easier to chew. As the name indicates, soda bread substitutes baking soda for yeast; the baking soda reacts chemically with the lactic acid in sour milk or buttermilk to produce bubbles of carbon dioxide in the dough.
In your search for authentic Irish soda bread recipes, beware long lists of ingredients, as well as formulas enhanced with Irish whiskey, raisins, citrus zest, and other tomfoolery. A traditional recipe for Irish soda bread won't include much beyond flour, baking soda, salt and buttermilk as its basic ingredients. Should you encounter a recipe that calls for exotic, expensive, hard-to-find, or hard-to-prepare ingredients, it will likely be quite delicious; such a recipe, however, is at the same time wholly inauthentic. Happy baking, and Bon Appetit!