Shamrocks have been symbolic of many things over the years. According to legend, the shamrock was a sacred plant to the Druids of Ireland because its leaves formed a triad, and three was a mystical number in the Celtic religion, as in many others.
St. Patrick used the shamrock in the 5th century to illustrate the doctrine of the Holy Trinity as he introduced Christianity to Ireland.
The shamrock became symbolic in other ways as time went on. In the 19th century it became a symbol of rebellion, and anyone wearing it risked death by hanging. It was this period that spawned the phrase "the wearin' o' the green."
Today, the shamrock is the most recognized symbol of the Irish, especially on St. Patrick's Day, when all over the world, everyone is Irish for a day!
The original Irish shamrock is said by many authorities to be none other than white clover, a common lawn weed originally native to Ireland. Occasionally, a fourth leaflet will appear, making a "four-leaf clover," said to bring good luck to the person who discovers it.