Long ago, when Ireland was the land of Druids, there was a great bishop, Patrick by name, who came to teach the word of God throughout the country...This saint, for he was indeed a saint, was well loved everywhere he went. One day, however, a group of his followers cam to him and admitted that it was difficult for them to believe in the doctrine of the Holy Trinity.
Saint Patrick reflected a moment and then, stooping down, he plucked a leaf from the shamrock and held it before them, bidding them to behold the living example of the "Three-in-One." The simple beauty of this explanation convinced these skeptics, and from that day the shamrock has been revered throughout Ireland.
~source, Irish Blessings