(as found online)
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 St. Patrick—for he was indeed a saint—was well loved everywhere he went. One day, however, a group of his followers came to him and admitted that it was difficult for them to believe in the doctrine of the Holy Trinity.
St. 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." St. Patrick used the shamrock to explain the concept of the Holy Trinity of the Father, the Son, and the Holy Spirit. The simple beauty of this explanation convinced these skeptics, and from that day the shamrock has been revered throughout Ireland.