A lot of people on this forum underestimate the power of faith in history.
Myself included. Meanwhile, you overestimate that power. Positions cleared out, then?
but without faith things like the Reformation, the Crusades, etc. would never have happened.
The Crusades were hardly
caused by religion, they were first and foremost caused by the Middle Eastern geopolitical situation. Religion seriously came into all this a bit later.
Deep down the Reformation was a religious movement, its leaders supported it for religious reasons, and the masses flocked to it for religious reasons.
Define "its leaders", please. If you mean the religious leaders that tried and failed a thousand times previously until finally they found coincidental support of POLITICAL leaders who had actually achieved something, yes, yes they were.
Likewise, in the Middle Ages, the Scandinavians nor anyone else, would have seperated Church and State.
Not QUITE separated. But, again, the Scandinavians distanced the Church - or, at least, Rome - from power as much as possible in OTL. That is technically what I meant; I suppose that calling it secularism is quite an exaggeration, sorry.
It would seem that you stubbornly refuse to understand what I and Symphony are getting at. WHAT MARTIN LUTHER?
I also agree that politically Ireland would remain isolated
Politically it actually wouldn't necessary be all that isolated; it will inevitably maintain SOME degree of contact with the outside word. Culturally and religiously, it is far more isolated, at least after a while.
I suspect it might go like this - after the initial flurry of the missionaries that you had suggested, there would be furious theological debates, a breakdown and a general reaction against them in the greater Catholic world. Angry mobs, inquisitions and so forth. Eventually the two cultural worlds - that of Ireland, and that of the rest of Europe - would close to each other (well, ofcourse not completely, that is virtually impossible, but for all purposes). The Irish would crush Latinists in their lands, the Catholics would fight the Hibernian Heresy. Bilateral hatred and so forth.
would also imagine a largely very well educated Irish populace
Was it very educated in OTL at the highest point of the Celtic Rite? I rather doubt the likelihood of Ireland becoming the focus of a renaissance; if anything, from the zenith (whether political, economic or cultural), there is an inevitable reccesssion to the nadir.
Also, why do you assume that the Celtic Rite will not at all change after centuries of being left to its own ways? Or the Roman Rite, for that matter? I put it to you that this world will have a radically different religious history. The very survival of the Celtic Rite and the subsequent war of theologies is bound to utterly change everything. In what ways exactly is up to debate. There still might be a Reformation, but to assume that it would be similar to the OTL one at all is nonsense, as the Roman Church too will be completely different from OTL.
With the decentralized hierarchy, a much smaller populace to work on and decidedly liberal (compared to Rome) theology makes corruption a lot harder.
Not at all. Firstly, decentralized hierarchy is but natural for a young movement; and it is also natural for it to grow more centralized. A much smaller population only helps; its far easier to control. As for liberalism, it too is natural for younger cultures and spiritual movements; it simply hadn't enough time to move towards a more consolidated, and thus conservative, model. Also, why the assumption that liberalism and corruption don't fit? Corruption goes with everything.
It was inevitable that someone would try to fix the ugly beast that the Catholic Church had been; there were plenty of movements that would support that.
Oh, someone would try alright, but the chances of it going practically in the same way as the OTL Reformation are near to nil. Why won't it, say, be a succesful reform WITHIN the Church itself? Or a heresy brutally rooted out, complete with a prolonged Dark Age? Or something else. The one thing I don't see as likely is Martin Luther, defying common sense, genetics and the butterfly effect. He was a pretty defiant man, I know, but that can only get you this far...
There were far earlier antagonists as well: John Wycliffe and the Lollards in the 14th century, Jan Hus and the Hussites in the 15th century, etc.
Almost from the Great Schism there was a general malcontent with the rule of the Catholic Church
The PoD is 11th century. After the Great Schism, the Catholic Church was indeed in crisis that demanded reform from above or revolution from below (in our world, a little of both happened). But the PoD is BEFORE the Great Schism, and the chances of the Great Schism happening in this world, with a radicall different political and theological history, are not very high. That event in itself was a bit of a fluke...