The shamrock, or trefoil (three-leafed), is a type of small herb with leaves made up of three leaflets and belongs to the family "Leguminosae."
Shamrock is actually a common name for any of several three-leafed clovers native to Ireland, including the white clover, red clover and black medic. However it is the green shamrock, or trefoil, that is the National symbol of Ireland.
The Shamrock, at one time called the "Seamroy," symbolizes the Holy Trinity. Before the Christian era, it was a sacred plant of the Druids of Ireland because its leaves formed a triad. "Three" was a mystical and sacred number to the Druids and the ancient Irish (Celtic) religion. It may have represented totality: past, present, and future; behind, before, and here; sky, earth, and underworld.
According to legend, St. Patrick, preaching in the open air on the doctrine of the Trinity, is said to have illustrated the existence of the "Three-In-One" by plucking a shamrock from the grass growing at his feet and showing it to his congregation. The simple beauty of this explanation convinced his congregation, who had been skeptical, and from that day the shamrock has been revered throughout Ireland.
The legend of the shamrock is also connected with that of the banishment of the serpent tribe from Ireland by a tradition that snakes are never seen on trefoil and that it is a remedy against the stings of snakes and scorpions.