There are many legends and stories associated with Saint Patrick. Probably the two most common of these are the stories to do with snakes in Ireland and Patrick's use of the shamrock.
It is common knowledge that there are no snakes in Ireland, and that the reason for this is that Saint Patrick fasted and prayed for many days and nights on Croagh Patrick for God to give him power over the serpents allowing him to command them to leave Ireland forever.
The real truth is that there are only two places in the temperate world which do not have snakes. One is New Zealand, the other is Ireland, and the likelihood is that there never have been any snakes in Ireland. The legend probably came about because in the Celtic tradition the horned serpent represents Cernunnos, god of fertility and virility. So Patrick banishes the "snake god" rather than actual snakes, because we never had any snakes in reality. This story may also have come about because the serpent was Satan "the great dragon... that old serpent called the devil and Satan" (Revs: 12,9).
It is also believed that when Saint Patrick tried to explain the Trinity to the early Irish pagans, they found it difficult to understand as they had previously believed in many gods. In trying to explain how there were three persons in one God, he used the shamrock with its three leaves but one plant to communicate the idea. This is why the shamrock is so important. It is also why so many people are mistaken in associating a "four-leafed clover" with Saint Patrick's Day.
Soon after he returned to Ireland, Patrick and his followers decided to celebrate Easter near Tara, because it was the greatest kingdom among the Irish tribes which practised idolatry. Tara was filled with many local kings, generals, nobility, and druids who were attending the pagan feast of Bealtaine which coincided with Easter that year. Patrick encamped in full view of the castle to celebrate the Resurrection of his Christ by lighting a Paschal Fire. On the eve of the pagan festival it was the custom, upon penalty of death, that the high king should light the first bonfire before any others in the land. Patrick, however, had kindled a great Paschal Fire which gleamed through the darkness to the inhabitants of the plain as well as all those gathered at the top of Tara. The fire was observed from the Hill of Tara and the druids told the enraged king Laoghaire that unless the offending fire was extinguished that same night, it would never be put out. A contest between Patrick and the druids ensued, the druids causing unnatural storms of thunder and snow. Patrick overcame all their spells, and when one druid attempted to use fire against Patrick, the druid himself was burnt to death. Patrick faced King Laoghaire and declared, "Unless you believe now, you will soon die, for God's wrath will come down upon your head." Reluctantly, the High King and his followers converted to Christianity.
On another occasion after Patrick had returned to Ireland, he asked King Daire for a site on a lofty hill in Macha. The pagan king was not ill-disposed towards Christianity and he granted Patrick a site for a monastic establishment. Daire did not give Patrick the lofty site but granted him land on the eastern foot of a hill, under the hill of Macha. When one of Daire's horses strayed into the land belonging to the holy sanctuary, Patrick was offended by a brute animal disturbing the sanctuary dedicated to God and he killed the horse. Daire learned of the horse's death and ordered his men to kill Patrick. The moment he issued this command, Daire was suddenly ill. His men asked Patrick for a remedy. Patrick gave them holy water which they sprinkled on the dead horse and restored it to life. The water also restored Daire to good health. Daire visited Patrick and gave him a precious bronze cauldron. Patrick's response was "I thank you" in Latin. Daire expected a greater display of gratitude and when he returned home, he ordered his men to retrieve the gift from the unappreciative Patrick. When the servants returned with the cauldron, they told Daire that Patrick had again replied "I thank you". The king was so impressed by the steadfast Patrick that he brought the cauldron back, and granted Patrick the land on the hill which he had originally requested.
