According to legend, Saint Patrick used the shamrock-- a three-leafed clover -- to explain the Holy Trinity to pre-Christian Irish. The leaf of this plant is now the national symbol of Ireland.

According to Wikipedia:

Saint Patrick BLUE was the color long-associated with Saint Patrick's Day. GREEN, the color that is now most widely associated with Ireland, Irish people, and with St. Patrick's Day, may have gained its prominence through the phrase "the wearing of the green," meaning to wear a shamrock on one's clothing. At many times in Irish history, to do so was seen as a sign of Irish nationalism or loyalty to the Roman Catholic faith. The wearing of and display of shamrocks and shamrock-inspired designs have become a ubiquitous feature of the saint's holiday. The change to Ireland's association with green rather than blue probably began around the 1750s.

For a simple leaf-pressing method, go here.

Happy St. Patrick's Day