As it relates to St. Patrick's Day in a religious sense, the shamrock carries significance for its three leaves, which represent the Holy Trinity of Father, Son and Holy Spirit. As a holiday celebrating Irish cultural heritage, however, the St. Patrick's Day shamrock is related to the "wearing of the green." This made the shamrock into a symbol of rebellion against governments attempting to control Ireland during the late 18th century, turning the simple green leaf into a symbol of Irish identity and patriotism.
As the shamrock became an increasingly popular symbol of Irish national identity, it likely became more closely associated with the secularized celebration of St. Patrick's Day. However, the shamrock was closely associated with images of St. Patrick himself before the holiday became a major international event. For example, coins dating back to the 17th century show an image of St. Patrick holding a shamrock, which clearly displays with three distinct leaves. This connection with St. Patrick was the earliest use of the shamrock as a symbol for the holiday celebrating his religious accomplishments.