Patrick (Patricius) was 16 years old in 405 when Irish invaders pillaged his British village and captured him, taking him to Ireland where he was made a slave by a cruel chieftain Milchu. Forced to tend sheep in Ireland, he was given ample amounts of time to pray to a God he had taken for granted in the ease of his youth. He turned to Him for salvation. He was told in a dream how to escape, and did so after six years of captivity.
What is incredible about Patrick is that he eventually returned to Ireland as a missionary, with Milchu supposedly one of his first converts. He took many of the celtic pagan rituals and symbols and converted them to Christian ones, such as the bonfires and celtic crosses.
Why a shamrock? Patrick ingeniously used the three-leaf clover as a way to demonstrate the trinity: Father, Son, and Holy Spirit.
Wake Up Call
I arise today
Through God's strength to direct me,
God's might to uphold me,
God's wisdom to guide me,
God's eye to look before,
God's ear to hear me,
God's word to speak to me,
God's hand to guard me,
God's way to lie before me,
God's shield to protect me,
God's hosts to save me from snares of devils.
From temptation of vices,
From everyone who shall wish me ill,
Afar and anear,
Alone and in a multitude.
—Attributed to St. Patrick, from "The Deer's Cry" (St. Patrick's Breastplate)
Read Patrick's only authentic literary remains The "Confessio" of St. Patrick here.