I was surprised last March that one of the teachers gave a talk to the kids about how St. Patrick was not a Catholic.
I had never before heard such a suggestion, but apparently it's a sore point with some Evangelicals.
In case it interests you, Jimmy Akin, a Catholic apologist, presents some evidence here that strongly suggests that Patrick was in communion with Rome: http://jimmyakin.com/was-st-patrick-catholic
The idea of "Roman" Christianity had not taken hold until Gregory the Great, and especially his successor, Sabinius, who formally accepted the supremacy over all the bishops.
The conflict between the Celtic and Roman churches came later under Augustine of Canterbury, who required the Celtic churches to adopt the Roman rite, abandon their old Italic Bible translations, and adopt the Vulgate. That stirred up quite a kerfuffle.
It's based on the different manuscript copies of the Confessio and the Epistola (or Letter) to Coroticus, both of which are acknowledged as being written by the man himself.