According to Baptist Press:
In William Cathcart’s “Baptist Encyclopedia,” a catalog of Baptist doctrines, organizations and individuals, there is a surprising entry under the letter P: “Patrick, Saint, the Apostle of Ireland.”
Two contemporary Baptist historians told Baptist Press that Patrick was not, in fact, a nascent Baptist, but those who sought to claim him as one rightly noted the error of associating him with the modern Roman Catholic Church.
“There is a romance about [Patrick],” said Michael Haykin a Southern Baptist Theological Seminary professor who has written a book about Patrick. “During the 19th century, with the rise of Landmarkism, there was a desire to claim as many people as Baptists as possible. In some respects, Patrick falls easily into that [classification]. He was almost definitely baptizing people by immersion. But he was almost definitely not a congregationalist — which we would view as central to being a Baptist.”