Christianity in Hibernia
Right after the invasion, like in neighboring Britannia, Hibernia had very few Christians within its boundaries. Roman officials worked with native associates to focus on converting the population to some sort of local/Roman hybrid of polytheism, or at least worship of the Emperors. Since most of the early "Roman" settlers were recently retired legionaries and auxiliaries with newly gained citizenship or Romano-Briton, or even Romanized British Celts, Christianity didn't take off in Hibernia for a long time. In fact there are no records of Christians until the mid-2nd century, and even then they were often persecuted, despite Imperial laws passed by Trajan, Hadrian and Antoninus Pius.
Those few Christians increased in number greatly as the Plague of Verus hit, as refugees from the Plague in Italy, Southern Gaul and Hispania, and refugees from the fighting in closer-by Northern Gaul and Germania fled north to the stable provinces of Hibernia and Brittania. According to Marcus Flavius's The Plague, he outlines the extent of immigration saying "By the time the Emperor came close to finishing the reconquest, the Hibernian Governor at the time, Publius Pinarius, was forced to tell the Harbormasters in Valeria and Flaviapolis to turn refugees from [Germania] away, as the Governor worried of urban overcrowding."
But the mark of the refugees was made. Christianity spread like wildfire across Flaviapolis in particular, as the double hit of Italian and Southern Gallic refugees who had converted, and a larger groop of urban poor and homeless because of them hit the city. While Christianity was legal at the time, Governor Pinarius worried that it would harm Imperial control somewhat. The Governor used military force in Southern Hibernia by calling out auxiliaries to prevent new Christian missionaries from infiltrating the Hibernian countryside, where the poor Celtic farmers would be sympathic to Christianity, with no particular zeal for the Roman pantheon their patrons were forcing down their throats.
Several men, known as the "Martyrs of Flaviapolis", snuck through fields and pastures to get past the Governor's barricades. They were often caught by local authorities, who killed them for "disturbing the peace". These men were venerated by Christians for generations to come. Hibernia received its first Bishop in 226, coming from a boost in the church's size with a period of economic strife.
As Caeso Julius Constantius rose to the position of Governor of Hibernia, he became the fifth Christian governor of a province in 300. He loosed many prior restrictions on Christians, and became an active member of the First Council of Caesaraugusta, one of the major early Christian councils, and after resigned to become Hibernian bishop. Governor Constantius became a very scholarly bishop, writing down the History of the Hibernian Church, a work sadly lost to us today, and eventually became a saint, St. Caeso of Hibernia, along with the five known Martyrs of Flaviapolis, one of the seven Hibernian saints.
As the Roman Empire collapsed in the mid and late 500s, the last Hibernian Governor made himself King of Hibernia as the Germanic hordes of Angles, Saxons and Frisians conquered neighboring Britain, the first Kings made themselves very militarized, to scare off the the Germans to the east, with a few even making military incursions into Britain's southwest. As the threat grew more distant when the neighboring Frisians became settled in Cymberland. The seventh Hibernian saint, St. King Trajan IX sent many missionaries among the Picts of Caledonia, the Frisians of Cymberland and the Angles and Saxons of Anglosaxony, where Christianity took off strongly.