In written English, the first reference to the Shamrock dates from 1571, and in written Irish, as seamrog, from 1707. As a badge to be worn on the lapel on the Saint's feast day, it is referred to for the first time as late as 1681. The
Shamrock was used as an emblem by the Irish Volunteers in the era of Grattan's Parliament in the 1770's, before '98 and The Act of Union. So rebellious did the wearing of the Shamrock eventually appear, that in Queen
Victoria's time Irish regiments were forbidden to display it. At that time it became the custom for civilians to wear a little paper cross colored red and green.
As a symbol of Ireland it has long been integrated into the symbol of the United Kingdom, along with the Rose, the Thistle and the Leek of England, Scotland and Wales. So today, on St. Patrick's Day, a member of the British Royal Family presents Shamrock to the Irish Guards regiment of the British Army.

THE MAGIC SHAMROCK 

Three is Ireland's magic number. Hence the Shamrock. Crone, Mother and Virgin. Love, Velour and Wit.. Faith, Hope and Charity. Father, Son and Holy Spirit. Numbers played an important role in Celtic symbolism. Three was the most sacred and magical number. It multiplies to nine, which is sacred to Brigit. Three may have signified totality: past, present and future OR behind, before and here OR sky, earth and underworld. Everything good in Ireland comes in threes. 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. "Three accomplishments well regarded in Ireland: a clever verse, music on the harp, the art of shaving faces."