The shamrock is a young sprig of clover. However, not all clover species are shamrocks. A shamrock has three leaves. Clover with four leaves are clover, and can be a symbol of good luck, but cannot be called a shamrock.
Shamrock was a sacred plant to the Druids of Ireland because its leaves formed a triad, and three is a mystical number. It is an (unofficial?) symbol of Ireland. The official emblem of Ireland is the Celtic Harp.
The custom of wearing a shamrock dates back to the 17th and 18th centuries. In 1900 Queen Victoria authorised the wearing of the shamrock on their uniform to celebrate St. Patrick for all Irish regiments as a gesture of gratitude for their service during the Boer War.
Patrick was born around A.D. 390 to an aristocratic Christian family. At 16, he was kidnapped and sent as a slave to Ireland. He escaped. returned to Britain on a pirate ship, was reunited with his family then returned to Ireland to convert the Irish to Christianity. After he died on March 17, 461, the missionary Patrick was largely forgotten. However, mythology grew about him until he was honored to become St. Patrick is the patron saint of Ireland.
St Patricks Day is celebrated on 17th march and until the 1970s, was a minor religious holiday. “St. Patrick’s Day was basically invented in America by Irish-Americans,” says classics professor Philip Freeman of Luther College in Iowa.
St. Patrick is thought to have used clover to represent the Holy Trinity when he took Christianity to Ireland.
Legend has it that St. Patrick also drove all the snakes out of Ireland, and saved the Irish people from a plague of reptiles and demons. This is disputed by Philip Freeman in his book St. Patrick of Ireland: A Biography.
Philip Freeman points out that although there are no snakes on the island today, there never were snakes in Ireland. Being surrounded by icy ocean waters it was too cold for snakes to migrate there. Instead Freeman suggests “when Patrick drives the snakes out of Ireland, it is symbolically saying he drove the old, evil, pagan ways out of Ireland [and] brought in a new age.”