Before the Christian era the Shamrock was a sacred plant of the Druids of Ireland because its leaves formed a triad. Numbers played an important role in Celtic symbolism. Three was the most sacred and magical number. It multiplies to nine, which is sacred to Brigid. Three signified totality: past, present and future / behind, before and here / sky, earth and underworld. Three is Ireland's magic number. Hence the Shamrock.
The rhythm of story telling in the Irish tradition is based on threefold repetition. This achieves both intensification and exaggeration. Even today in quality pub talk, a raconteur can rarely resist a third adjective, especially if it means stretching a point.
Legend suggests that the Shamrock was used by St. Patrick in the fifth century to demonstrate the meaning of the Trinity. Found on Irish medieval tombs and on old copper coins, it is known as St. Patrick's money. The plant was reputed to have mystic powers... the leaves standing upright to warn of an approaching storm.
Only one thing is certain about the shamrock, worn by millions on St. Patrick's Day. The word is derived from the Irish 'seamrog', meaning 'summer plant', and it remains Ireland's most famous symbol.