In ancient pagan Ireland, the number three had special siginifcance, since many "triple deities" were worshipped. The triple god Brigid was depicted simutaneously as Poetess, Smith (as in industry), and Healer. Parallel mythology depicts Boann, Brigid and Ceridwen as the Triple Goddess of Creativity, Inspiration and Vision.
Other popular myths cite the triplets or stages, of female life, Maiden, Mother and Crone. However, the true origins of "three-ness" are much more profound. Three moves one beyond the "one" (ourselves) and even the "two" of duality into the greater mystery "otherness". The Triskel (right), a three-fold Celtic spiral appears carved into even the most ancient artifacts, not only in Ireland, but in many other parts of the world. The Celtic Triadic was a three-part verse, an ancient Celtic "haiku" of sorts, which was easy for children to remember and pass on wisdom in oral tradition or song.
When St. Patrick came along in the 4th century AD, he popularized the three-leaf shamrock as a Christian metaphor in his teachings to explain the Holy Trinity. Ireland has come to use the shamrock as a symbol of national identity.