The shamrock (three-leafed clover) was chosen as Ireland's national emblem because of the legend that St. Patrick told about the Trinity. The Trinity is the idea that God is really three-in-one: The Father, The Son and The Holy Spirit.
The Irish have considered shamrocks as good-luck symbols since earliest times, and today people of many other nationalities also believe they bring good luck.
It also represents a star that shines. Green colour of shamrock means growth, is the symbolic color of all that can develop and prosper.