St. Patrick used the shamrock to explain the Holy Trinity: One leaf, three parts; One God, three persons.
Patrick uses shamrock in an illustrative parable
Legend (dating to 1726, according to the OED) also credits St. Patrick with teaching the Irish about the doctrine of the Holy Trinity by showing people the shamrock, a three-leafed plant, using it to illustrate the Christian teaching of how three persons in one God. For this reason, shamrocks are a central symbol for St Patrick’s Day.
The shamrock had been seen as sacred in the pre-Christian days in Ireland. Due to its green color and overall shape, many viewed it as representing rebirth and eternal life. Three was a sacred number in the pagan religion and there were a number of “Triple Goddesses” in ancient Ireland, including Brigid, Ériu, andthe Morrigan.