Legend has it that St. Patrick would use the shamrock to explain the Trinity - Father, Son and Holy Spirit. The word "shamrock" comes from the Gaelic word "seamrog" (In irish, it means "summer plant") meaning "trefoil" (three leafed) or "little clover". In Arabia, it is called shamrakh. It was a sacred emblem in Iran and to the Persian triads. It is also a sacred plant among the Druids. Shamrock is the national flower of Ireland.
Via S.O. Hawkins