The traditional Irish bread commonly associated with St. Patrick’s Day is soda bread.
Irish soda bread is a quick bread made with flour, baking soda, salt and buttermilk. The acidity in the buttermilk reacts with sodium bicarbonate, producing carbon dioxide bubbles that help the loaf rise. Unlike yeast bread, it does not need a long fermentation or proving period.
The bread developed in Ireland during the early 19th century, when baking soda offered a cheap and dependable leavening method for local soft wheat, which was less suited to making lofty yeast loaves. By the 1860s, soda bread was widely baked in Irish homes and had become an everyday staple.
Recipes vary by region. Plain white soda bread is generally savoury, while wholemeal versions are often called brown bread or wheaten bread. American versions may contain sugar, raisins or caraway seeds, but those additions are not required for traditional Irish soda bread. Brown bread is therefore a related style, not the clearest answer to this question.