It's total BS. Ireland's Christianization was incredibly peaceful compared to conversions elsewhere (there were no martyrs in Ireland's transition to Christianity) and actually preserved many aspects of pagan belief and superstition, rather than rooting them out.

Christianity was actually introduced to Ireland even before St Patrick through Roman Britain, and evidently spread amongst the most politically powerless segments of Irish society; women and slaves. Indeed, Patrick wasn't even Ireland's first evangelist; a man named Palladius was sent to "the Irish believing in Christ" in 431 AD.

Christianity was adopted incredibly quickly in Ireland not because it was successful at rooting out and destroying pagan structures, but because it Christianized pagan beliefs and traditions. Healing wells and springs associated with deities were now associated with local saints, and Irish warriors now called upon Jesus, Saint Patrick or Saint Columba for victory in battle rather than their pagan gods. Christianity's success can be linked to its widespread adoption by the Irish aristocracy, whose druids seem to have quickly become Christian clergy, and whose warrior-aristocrats were likely convinced by its superiority as a form of military magic.

Irish pagans weren't philosophically attached to their religion; they just used whatever magical forms of manipulation could work best for them. The belief in magical manipulation was central to the belief structures of all pre-Christian Europeans and survived in popular belief until the Modern period. When Christianity justified those forms of magic by attributing them to Jesus or God, there was no real barrier for an Irish pagan to become an Irish Christian - there was less of a cataclysmic break from the past than a blended transition where elements of old structures of belief survived because the clergy couldn't risk alienating the majority of the population.

As well, Christianity with its clear doctrine and corpus of literature, filled with apparently magical events, could justly assert itself as a fundamentally more powerful way of attaining magical power. Drawing on Old Testament God's ability to destroy cities and flood the earth, Christianity was evidently more powerful than paganism whose gods were more localized and had more diffused power. Why would you worship a bunch of gods with different, limited powers when you could worship one supreme god who could give you all your benefits at once? With Christianity, one could harness a single god's power for prowess in battle, fertility for their crops, protection from weather or plague etc. instead of having to supplicate a whole bunch of different deities and spirits. Not only did Christianity simplify the process of gaining magical powers, but it also had a whole corpus of events in the Bible that could be used as proof of the new Gods' power, while Ireland's pre-Christian belief system had no doctrine or dogma and thus lacked Christianity's persuasive power.

Pagans were never subjugated in Ireland Christianization. Their beliefs were actually adopted by the Christian church (in fact, almost everything we know from pre-Christian belief in Ireland comes from Christian texts!) and they willingly adopted it because of its perceived magical power.