On this day in 1631, the Catholic Church established a Feast Day honoring St. Patrick, the Patron Saint of Ireland, who'd died in the mid-400s.
As the legend goes, St. Patrick's name was Maewyn Succat and when he was 16, Irish pirates attacked his family's estate (in lowland Scotland or Whales). The young Succat was kidnaped and sold into slavery in Ireland. Six years later he managed to escape. He joined the Catholic Church and studied as a missionary, taking the name Patricius (or Patrick) meaning "father figure." As a Bishop, Succat returned to Ireland after having a dream calling him back to spread Christianity among the Druids. Apparently he had a lot of luck!
The shamrock didn't become a symbol of St. Patrick's Day until the 1720s when the church gave an official plant to all saints and the color green wasn't associated until eighty years later, during the Irish Rebellion (1798). Before that, the color was blue (a main color in both the royal court and ancient Irish flags). Since the British wore red, the Irish chose the opposite color, green. The song “The Wearing of the Green” during the rebellion, "cemented the color’s relevance in Irish history."
As far as the drinking goes, up until the 1900s Ireland had a law that kept everything closed on St. Patrick's Day, including pubs. And the green beer? You can thank Budweiser's 1980s marketing team for that one.
Sources:https://www.irish-genealogy-toolkit.com/history-of-st-patrick.html and http://time.com/4261456/st-patrick-day-2016-history-real-saint/
Various staff and volunteer writers.