That baffles some people because they equate St. Patrick with Ireland and most of Ireland is Catholic.
He was probably a Scotsman, and his church loyalties were with the ancient church of Britain.
So, how did he get to be affiliated with Ireland?
Born in the late 300's, Patrick was enslaved by the Irish as a teen. Made to work as a shepherd, he had lots of time to think and pray and find God. He finally escaped Ireland and headed back to Britain. He wanted to stay there, and his parents wanted him to stay there, but God kept sending him dreams and visions of Irish people who were begging him to come back and teach God's ways. Eventually, he managed to get back to Ireland and he spent 33 years there teaching and preaching the Word of God.
He died on March 17, 465.
You can find some more good information and Patrick's own Confession here. (Scroll down a bit for the Confession, it's after the political ramblings...)
Happy St. Patrick's Day!