This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Christian mission takes organized root in Ireland beginning in 431 CE, when Pope Celestine sent Palladius as first bishop to those in Ireland already believing in Christ, the only contemporarily attested date in the whole story. Patrick, whose name is now far better remembered than Palladius's, is a separate and much less securely dated figure: his traditional arrival in 432 rests on a retrospective annalistic reckoning rather than any independent record, and his own death is disputed between 461 or 462 and as late as 492 or 493.
Whether the work of one missionary or two, the period running from 431 into the later fifth century is when organized Christian mission took hold across Ireland. Patrick's own surviving writings, among the earliest personal testimony in this whole story of Christianity's spread, describe preaching, baptising and ordaining clergy directly in his own words, evidence of a working mission rather than a later legend alone.
The Armagh centred tradition that grew up around Patrick became the organizing myth of the later Irish church, outlasting the more precisely dated Palladius in memory even where the record cannot always confirm the detail. What this changed for Christianity's spread was the planting, across this sixty year span, of an organized clergy and church structure in Ireland, carrying the faith to an island the Roman Empire itself had never governed.