Original Saint patrick's day harp & lady Postcard
On St Patrick's Day it is customary to wear shamrocks and/or green clothing or accessories (the "wearing of the green"). St Patrick is said to have used the shamrock, a three-leaved plant, to explain the Holy Trinity to the pagan Irish. This story first appears in writing in 1726, though it may be older. In pagan Ireland, three was a significant number and the Irish had many triple deities, a fact that may have aided St Patrick in his evangelisation efforts. Patricia Monaghan says there is no evidence that the shamrock was sacred to the pagan Irish. However, Jack Santino speculates that it may have represented the regenerative powers of nature, and was recast in a Christian context—icons of St Patrick often depict the saint "with a cross in one hand and a sprig of shamrocks in the other". Roger Homan writes, "We can perhaps see St Patrick drawing upon the visual concept of the triskele when he uses the shamrock to explain the Trinity".
The colour green has been associated with Ireland since at least the 1640s, when the green harp flag was used by the Irish Catholic Confederation. Green ribbons and shamrocks have been worn on St Patrick's Day since at least the 1680s. The Friendly Brothers of St Patrick, an Irish fraternity founded in about 1750, adopted green as its colour. However, when the Order of St. Patrick—an Anglo-Irish chivalric order—was founded in 1783 it adopted blue as its colour, which led to blue being associated with St Patrick. During the 1790s, green would become associated with Irish nationalism, due to its use by the United Irishmen. This was a republican organisation—led mostly by Protestants but with many Catholic members—who launched a rebellion in 1798 against British rule. The phrase "wearing of the green" comes from a song of the same name, which laments United Irishmen supporters being persecuted for wearing green. Throughout the 19th and 20th centuries, the colour green and its association with St Patrick's Day grew.
The wearing of the 'St Patrick's Day Cross' was also a popular custom in Ireland until the early 20th century. These were a Celtic Christian cross made of paper that was "covered with silk or ribbon of different colours, and a bunch or rosette of green silk in the centre".