May the road rise to meet you, 

May the wind be always at your back.

May the sun shine warm upon your face,

The rains fall soft upon your fields and,

Until we meet again,

May God hold you in the palm of His hand.

From the History Channel's website:

Leprechauns

The original Irish name for these figures of folklore is "lobaircin," meaning "small-bodied fellow." Belief in leprechauns probably stems from Celtic belief in fairies, tiny men and women who could use their magical powers to serve good or evil. In Celtic folktales, leprechauns were cranky souls, responsible for mending the shoes of the other fairies. Though only minor figures in Celtic folklore, leprechauns were known for their trickery, which they often used to protect their much-fabled treasure. Leprechauns had nothing to do with St. Patrick or the celebration of St. Patrick's Day, a Catholic holy day.

Corned Beef & Cabbage

Each year, thousands of Irish Americans gather with their loved ones on St. Patrick's Day to share a "traditional" meal of corned beef and cabbage. Though cabbage has long been an Irish food, corned beef only began to be associated with St. Patrick's Day at the turn of the century. Irish immigrants living on New York City's Lower East Side substituted corned beef for their traditional dish of Irish bacon to save money. They learned about the cheaper alternative from their Jewish neighbors.

Music

Music is often associated with St. Patrick's Day—and Irish culture in general. From ancient days of the Celts, music has always been an important part of Irish life. The Celts had an oral culture, where religion, legend, and history were passed from one generation to the next by way of stories and songs.

After being conquered by the English, and forbidden to speak their own language, the Irish, like other oppressed peoples, turned to music to help them remember important events and hold on to their heritage and history. As it often stirred emotion and helped to galvanize people, music was outlawed by the English. During her reign,Queen Elizabeth I even decreed that all artists and pipers were to be arrested and hanged on the spot.

Today, traditional Irish bands like The Chieftains, the Clancy Brothers, and Tommy Makem are gaining worldwide popularity. Their music is produced with instruments that have been used for centuries, including the fiddle, the uilleann pipes (a sort of elaborate bagpipe), the tin whistle (a sort of flute that is actually made of nickel-silver, brass, or aluminum), and the bodhran (an ancient type of framedrum that was traditionally used in warfare rather than music).

Shamrocks

A three-leafed clover, the shamrock is the national emblem of Ireland. Although it is widely believed that St. Patrick used the shamrock to illustrate the Christian doctrine of the trinity, this idea cannot be proven. In fact the first written mention of this story did not appear until nearly a thousand years after Patrick's death.

The shamrock, which was also called the "seamroy" by the Celts, was a sacred plant in ancient Ireland because it symbolized the rebirth of spring. By the seventeenth century, the shamrock had become a symbol of emerging Irish nationalism. As the English began to seize Irish land and make laws against the use of the Irish language and the practice of Catholicism, many Irish began to wear the shamrock as a symbol of their pride in their heritage and their displeasure with English rule.

The Snake

It has long been recounted that, during his mission in Ireland, St. Patrick once stood on a hilltop (which is now called Croagh Patrick), and with only a wooden staff by his side, banished all the snakes from Ireland. In fact, the island nation was never home to any snakes. The "banishing of the snakes" was really a metaphor for the eradication of pagan ideology from Ireland and the triumph of Christianity. Within two hundred years of Patrick's arrival, Ireland was completely Christianized.

The Celtic Cross

This enduring symbol was created when St. Patrick superimposed a sun, a powerful Irish symbol, onto a Christian cross.

For much more  - please visit The History Channel-St. Patrick's Day