Traditionally, the shamrock is said to have been used by Saint Patrick to illustrate the Christian belief of the Holy Trinity when Christianising Ireland in the 5th century.
The shamrock is so commonly seen growing on the island of Ireland, the very hue of the plant's chlorophyll is a symbol of Ireland itself - both in terms of the appearance of physical island; and as a representation the Irish culture, shaped by a long, historical accumulation, and integration, of diverse domestic and foreign influences. The low, dense, tangled, growing habit of shamrock plants creates a short, but expansive, mat of green, which colors the fields, hills, and forest edges of Ireland; hence the nickname, the Emerald Isle.
Symbolic shamrock meaning got its most popular reputation in Ireland around the fifth century.
During this time Saint Patrick was going about the business of introducing and establishing Christianity among the people.
St. Patrick used the shamrock to demonstrate his passion for the Christian faith to the people. This was a stroke of brilliance, because very often it is nature that offers the most profound and meaningful lessons.
St. Patrick must have understood this as legend indicates he utilized the three leafed clover as an illustration of the Christian holy trinity. St. Patrick tuned into the natural triad of the shamrock, and as clover was so prevalent on the Irish hillsides, the shamrock seemed a natural, logical visual-aid for making his point. Patrick was passionate about sharing his faith with the people of Ireland, and cleverly found associated each leaf of the clover with an aspect of the Holy Trinity.