Mmm. For the POD, I would see well a failure of gaelic invasions. Many missionars were scottish or brittons, probably from irish origin.
If you have the Germanic peoples (Angles, Frisons, Saxons, Franks) being less sucessful in their conquest, Brittons could be more interested to keeping the Christian net on their island than convert Gaels, at least for a while.
Effects?
Nothing would change really, as Irish christianity was anyway very close to secular institutions, not forming a mantle on it. Probably that a later conversion, fully "roman" this time would have created some tributaries states in the eastern coast, as Charlemagne did with western slavic tribes.
So, western Ireland could stay pagan until the viking invasion at least, while the eastern part could known christian or christianized elites ruling on a pagan population.