I feel compelled to respond. I just don't believe this is scenario is remotely practical to do in real life.
I am also particularly concerned that the Irish people in all their rich complexity, have been reduced to the status of weak, facile, eternal victims of the evil English oppressors which is both extraordinary simplistic and the usual narrative of anybody with historical axes to grind. The Irish may have had terrible things done to them at various times, and yet helped perpetuate terrible wrongs on others around the world whilst serving in Queen Victoria's armies and navies. The great cities of Ireland, especially Dublin and Belfast benefited immensely from the one-sided trade-in cash crops and people that came from that. Make the Irish slaves and clean their hands. Conveniently ignoring the fact that Irishmen and women also helped liberate the world alongside their English cousins from the scourge of Nazism, Fascism and rampant Militarism whilst the Irish Free State sat back and thought only of its survival and then punished those who stepped up and did the right thing.
I have multiple identities, I am a liberal Catholic, a Lowland Scot, a Borderer, part of the Hungarian and Ukrainian Jewish diaspora and British and I have experienced Sectarianism. That requires me to be nuanced, thoughtful and critical. There are always multiple perspectives and difficult and uncomfortable truths. Throwing the potato famine around as a weapon helps no one, it DOES NOT bring the dead back to life and DOES NOT change the fact that a large part of Irish agriculture was unsustainable in the form practised in the 1840s. There were too many people living in the countryside sustained by subsistence farming that died - end of. The London administration ABSOLUTELY made it much worse than it needed to be, no dispute about that, but we also shouldn't underestimate the work done by Landowners and middle-class organisers who did recognise the problem and try to solve it. It's worth considering the weird consequence of the Highland Clearances here. The Highlands were very similar to Ireland in a lot of ways and had large Catholic populations (the
current data per 2011 census, note most Catholics live around Glasgow or in the Highlands), but after the Clearances happened - where the people Cleared were NOT enslaved - there were fewer people, conducting a different, more financially sustainable form of agriculture which paid for the building of infrastructure and market towns in the Highlands and had a more active professional and Factor-class who did act for the most part quite decisively in a lot of places when the blight reached Scotland. Fewer people died. It's very likely that without the Clearances the Famine in the Highlands would have been just as bad as it was in Ireland, which is both ironic and completely whack.
My point is that NOBODY in these islands comes out looking good. The Scots are just as likely to divide on each other based on Protestant vs Catholic BS or question each other's patriotism based on how they vote. We are not innocent victims. We just have a stronger neighbour who was not necessarily magnanimous with their power when we might have wished. Your question may as well have been "why didn't the Russians enslave Georgia/Poland/Finland/whoever?" Regardless of one's views on a tone of history or politics, we are stuck together in these islands and as such much live together with some semblance of pragmatism. Constantly waving each other's sins in each other's faces doesn't help us do that or build any sort of future together as neighbours and trade partners at the very least. It is doing the exact opposite that has done nothing but power hatreds and the kind of violence that was a dreadful fact of life for anybody who was born before Good Friday 1998.
There is an also HUGE difference between a dominant culture being total arses to their less powerful neighbours (which is indisputable) and a dominant culture enslaving their less powerful neighbours who look like them in the Early Modern or Modern eras. In any period after the printing press becomes an everyday thing, for a state to remain stable, there must at least be some consent from the Governed. The British Isles have seen a multitude of rebellions based on disaffected people over the centuries. I just don't believe there would be enough active support for this to do it. Cromwell is a trope at this point. He was a religious zealot and religious zealots are not the majority of ANY population. Additionally, poor communications and the long distances involved meant that it was much harder for rational minds to apply common sense to effectively counter the propaganda put out there to justify whatever they are doing - hell, it was challenging for the Allies to counter Nazi propaganda and present a straightforward narrative to the German peoples on what was being done in their name and that was the 1940s. And of course, who wants to admit that BOTH SIDES were committing war crimes. The Catholic Irish did terrible things to the Protestant Irish and vis versa, as they have continued to do until the present day fighting and re-fighting the same wars. Rather as we could look the other way on the Red Army raping its way across Germany because Germans=Nazis.
I don't think the Churches or what minimal education system there was would go along with it blindly. I also believe that you under-estimate the processes needed for a person to enslave people who look like them. The legal framework needed to do this alone would be INCREDIBLY complex and inevitably contradictory. We are not talking about discriminating against a group because you don't like them - just as an example, it took until 2004 for the British military to legalise same-gender sexual contact. We are talking about something FAR more profound than that. There would be widespread consequences for the rule of law and all society there, basically saying all people who are Catholic are not Human. Does that just mean in Ireland or over all of His/Her Majesty's domains? Have you considered the implications for the recusant population in the UK itself? Of course not, because in this scenario Irish=victims, English=evil. Or more accurately only SOME Irish are victims because Catholics are the ones who count and ALL English are evil.
This scenario relies on treating all English people as villains and as a monolith. They are not. Not all English people even at the time were ethnically English. I would dare say the Cornish would be only too aware of their own delicate position. The Huguenot population, the English Jewish population and the Recusants in England itself would also be feeling incredibly vulnerable. Some are fools or bigots, the VAST majority are just ordinary people who usually don't think about politics, but you better believe that probably changes in this scenario. Re-read
Mansfield Park, if you are familiar with the references that would have been common currency in Georgian society but are not now, Jane Austen was not subtle in her disapproval of Caribbean slavery, indeed, there is an entire tranche of post-colonial readings of that novel (there's a nice overview of the work on
Wikipedia as a starter, she packed A LOT into that book). She's a Vicar's daughter who is politically consistent with her class, has family ties to France and fell in love with an Irishman. But no, she and women like her are probably motivated to sign petitions against the evil committed so close to their shores and to people who are much the same as the ordinary agricultural worker her father preaches to every day and by Wedgwood china to support the cause when she can. She'll write a straight-up political book. If you thought
North and South was political, you ain't seen nothing yet when Mrs Elizabeth Gaskell puts pen to paper. Walter Scott's bibliography probably looks a lot different as well and he was Tory, his subject matter likely changes. Leading Whig ladies of the day coordinate the opposition whilst their husbands sit on Manumission Committees and attack it by legal and political means. They couldn't win the propaganda war on African slavery, they are NOT going to be more successful over Irish slavery.
Not every German a Nazi. Most were fellow travellers or sticking their heads down and getting on with life, yes, they CERTAINLY could have done A LOT more to protect their fellow men, but there was also a decent sides cadre of active anti-Nazis from a wide range of backgrounds and roles from individuals helping to protect their Jewish neighbours to Generals plotting to murder Hitler. The head of military intelligence Admiral Wilhelm Canaris is a very interesting example of a traditional conservative with views consistent for the time and the place but who saw the implications of the regime, didn't like it and decided to use the system to undermine it from the inside. He paid for this with his life, executed at the age of 58 years at Flossenburg concentration camp on the 9th April, as the Reich collapsed in on itself. I would be unsurprised if there aren't small groups of Ascendency types, with family estates or good professions who don't even up opposing the system from the inside based entirely on disagreeing with it. Let us not forget that the author of the hymn 'Amazing Grace' was a slave trader who changed his mind.
If creating this system requires a leap of logic, enforcing it and sustaining it seems impossible. But then again, I suspect logic wasn't quite the point, so hey....