The Feast of St. Patrick is celebrated by the Roman Catholic Church, the U.S.Episcopal Church, as a commemoration by the Evangelical Lutherans, and venerated in Orthodox Church. It is a public holiday in Ireland. The shamrock was used by St.Patrick to explain the Holy Trinity. In Ireland it is celebrated by families getting together for a meal. If the day falls on a Friday during Lent, observant Catholics receive dispensation to eat meat. If the feast day falls during Holy Week (and it does occasionally), the feast day is moved to avoid conflicting with the Holy Week calendar. A more recent occurrence are public festivals in Ireland and use of the day to promote Irish culture.

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Here is an old tune from the Emerald Isle, known as The Minstrel Boy. The full lyrics can be found here.The tune was quite popular (and still is) and the opening is often heard more than the full song:

The minstrel boy to the war is gone,
In the ranks of death you’ll find him;
His father’s sword he has girded on,
And his wild harp slung behind him;
“Land of Song!” said the warrior bard,
“Though all the world betrays thee,
One sword, at least, thy rights shall guard,
One faithful harp shall praise thee!”

The first is a wonderful rendition using Irish traditional musical instruments. And the second is from a more modern source (and set in the future) from Star Trek:The Next Generation episode The Wounded where the song has an important role. Chief O’Brien uses the tune to remind his old captain of his duty and what he has done.