Luck of the Irish! According to legend it was used by St. Patrick in explaining the doctrine of the Trinity; it is now used as the emblem of Ireland. An artificial or real shamrock leaf is customarily worn on St. Patrick's Day. The tradition of wearing Shamrock on Saint Patrick's Day can be traced back to the early 1700s. For good luck, it's usually included in the bouquet of an Irish bride, and also in the boutonniere of the groom.