When St. Patrick came back to Ireland as an ordained priest to convert the Irish, he used the local traditions in order to get the people to listen to him.
In order to explain the Holy Trinity to the local population he would pick a shamrock to show the three-in-one concept without confusing his audience.
The shamrock worked so well that the plant was used almost exclusively with St. Patrick for over 400 years. Many of the portraits of St. Patrick include him clutching a bushel of shamrocks.
St. Patrick’s Day, falling in the middle of Lent, was traditionally a relatively quiet affair that ended with the “drowning of the shamrock”. After mass, the men would go to the nearest tavern, toast St. Patrick and pluck the shamrock from their hat and drown it in their glass. When they were finished, they would pull the shamrock out of the glass and throw it over their left shoulder.
It wasn’t until the 19th century that the shamrock was no longer exclusively associated with St. Patrick and became adopted as a symbol for all of Ireland.