It is true that St. Patrick was never canonized. The reason for that is St. Patrick lived in the 5th century and the process we now know as canonization did not exist until centuries later. Up until that time holy men and women were declared saints on the local level and the local Bishop would add them to the local liturgical calendar. By the time the formal canonization process we know today came about St. Patrick was already renown as a Saint so there was no need for the process. Since St. Patrick has a feast day (March 17) in today's universal Church liturgical calendar you can rest easy that the Church truly considers him a saint.