The shamrock is the national flower of Ireland because of Saint Patrick. Credited with bringing Christianity to Ireland, Saint Patrick used the shamrock to explain the Christian belief in the Holy Trinity to the pagans of Ireland.
While historians lend credence to Saint Patrick's use of the shamrock, another legend associated with the patron saint of Ireland is purely mythical. Those with a casual knowledge of Saint Patrick likely know he is most commonly credited with driving the snakes out of Ireland.
However, snakes have never been indigenous to Ireland, and many historians note that Ireland was separated from the rest of the continent by the end of the Ice Age, which would have made it impossible for snakes to infest Ireland by the time Saint Patrick was born. The legend of Saint Patrick driving snakes from Ireland is likely rooted in the pagan practices that employed the use of serpent symbols during worship. So when Saint Patrick is credited with driving snakes from Ireland, this is likely just symbolic of his success at spreading Christianity and putting an end to pagan practices, including those that used likenesses of serpents.