The argument that the story of St. Patrick driving snakes out of Ireland is merely a legend is merely a myth. Snakes--or serpents--were a Druid symbol, and Patrick's missionary work in Ireland eventually led to the complete eradication of that pagan religion.
St. Patrick's Day is now associated generally with the Irish and shamrocks and the color green. That it has something to do with an actual saint is, unfortunately, less well known. Far be it from me to throw cold water on the traditional festivities (I wouldn't think of it). But before the parties tonight, read the incredible story of St. Patrick in his own words. You'll be glad you did.
Oh, and don't count on the real story of St. Patrick being being taught in your local public school.