The shamrock, not to be confused with the 4-leaf clover, is an important symbol in the Irish culture.
Legend has it that Saint Patrick, the patron saint of Ireland, has used the shamrock in the fields of Ireland to explain Trinity and convert pagans to Christianity. Three leaves, but still one plant.
Though not theologically flawless, this analogy still lives on & is used to teach children about the Trinity.
Happy St. Patrick's Day everyone :)