A shamrock is a clover that has three leaves and a traditional symbol of Ireland. In a Catholic legend, St. Patrick used a shamrock to illustrate the Holy Trinity of the Father, Son and Holy Spirit.BBC: The Truth Behind the Shamrock (March 17, 2004)
The ancient Druids believed that the shamrock was a sacred plant because its leaves formed a triad and three was considered a mystical number.BBC: The Truth Behind the Shamrock (March 17, 2004) Finding a four leaf clover is considered an omen of good luck. Shamrocks and four leaf covers are popular symbols of St. Patrick's Day.Washingtonpost.com: A Little Irish Luck Goes a Long Way (March 17, 2005)
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Wikipedia: Shamrock
Google Books: Irish Druids and Old Irish Religions
BBC: The Truth Behind the Shamrock (March 17, 2004)
FOXNews.com: Feds Seize Shamrock Seeds at Dulles Airport (March 16, 2009)
BBC News: Politicians Lead Annual Paddy's Exodus (March 16, 2009)
Royal Purple: The Meaning Behind the Shamrock (March 11, 2009)
Washingtonpost.com: A Little Irish Luck Goes a Long Way (March 17, 2005)