He wasn’t Irish. A Brit born in the 4th century, a third generation Christian, Patrick was the victim of an attack on his homeland and enslaved by the Irish when he was 16. He escaped 6 years later. After becoming a priest, he felt called by God to return to Ireland in order to win them for Christ. How could a Brit be so honored by the Irish? His ministry was really successful, because, according to Dana Robert (Kindle Locations 2006-2009):
Patrick's own writings give hints as to why his message appealed to the Irish. The first was the deep level of identification he felt with them. As do all good missionaries, he spent years among the Irish learning about their culture: he interpreted his enslavement as part of God's larger plan for his life task. He understood the Irish and knew their language. He spoke in idioms they understood, as is shown by the reference to his role as "hunter." He translated the deeper meanings of Christianity into Irish modes, helped no doubt by the Celtic roots he shared with them.
Patrick’s story is absolutely fascinating. And so is the missiological analysis on him by Dana Robert. I encourage you to read her chapter devoted to St. Patrick in Christian Mission: How Christianity Became a World Religion (2009).