this is one of the biggest misconceptions about slavery. people think slavery was about race; it wasn't, except in a very small time window in a very small area of the united states. what slavery was actually always about was religion.

why did the british enslave the irish? the reason is that they were catholic.

on the other hand, why were the ethiopians never enslaved, and not even colonized until the late nineteenth century? the reason is that when the portugese got there, they realized they were christians, and they consequently had no authority to enslave them. ethiopian christianity appears to be egyptian in origin, and to stem from the middle byzantine period.

why were the indigenous people of the americas enslaved by the spanish? because they weren't christians.

and, why were the slavs and balts of eastern europe enslaved by almost everybody, christians and muslims? because they converted to christianity relatively late. the word "slave" in english is derived not from a word about africans but from the continuing ethnonym for eastern european; slave and slav are the same word.

the history of slavery in the new world begins with a papal bull that gave the european explorers the explicit right to enslave non-christians:

We grant you [Kings of Spain and Portugal] by these present documents, with our Apostolic Authority, full and free permission to invade, search out, capture, and subjugate the Saracens and pagans and any other unbelievers and enemies of Christ wherever they may be, as well as their kingdoms, duchies, counties, principalities, and other property [...] and to reduce their persons into perpetual servitude.

that is from a statement issued by the pope in 1452, 40 years before colombus, but well after the circumnavigation of the cape. it was partially directed at africa.

however, what it was really directed at was the turks, who were in the process of conquering constantinople, which fell in 1453. the pope realized he would likely be targetted.

however, there's fundamentally a concept of reciprocity underlying it, as it was essentially a restatement of existing turkish policy. the era of dangerous turkish pirates raiding the european coasts (which ended only with the french invasion of algeria in the nineteenth century) was still a ways away, but (and this is contrary to a popular myth) there was a longstanding practice in the muslim world of enslaving just about anybody that wasn't muslim - and this included africans, europeans, indians, asians and really anybody else they can get their hands on. i think it's well known that there were large slave networks moving from africa into arabia as early as 1000 ce, but there were also networks moving from india and also from the ukraine. those fair-haired ukrainian women were a special prize for arab slave traders, as they fetched good money.

so, european slavery started in 1452 as a consequence of a papal bull, was based on religion (not race) and was actually an emulation of existing muslim practices, not an original innovation.

england, of course, went through a process called the reformation in ways that mainland europe didn't. in the process, it ended up with it's own church. it also went through a period of puritanical fundamentalism that culminated in a short dictatorship under the control of a fascist called oliver cromwell. throughout these various power struggles, catholics faced various types of persecution, over quite a long period; it wasn't the irish that were enslaved specifically, so much as anybody that was catholic, which included the irish. catholic emancipation was still a political issue in the uk until the late nineteenth century. while the authorities and legal statements shifted all over the place, the ultimate authority for slavery in england did come from the papal bull, even if it ended up with protestants enslaving catholics.

so, when the various european countries started transferring african slaves across the ocean, the legal justification for it was never that they were black, but always that they weren't christians. as mentioned: ethiopians were never enslaved, because they were already christians.

and, the reality is that there was a period where conversion to christianity was actually a way out of slavery.

eventually, however, the system began to collapse. there is of course a contradiction between christianity and slavery, whether the slaves are christian or not. the africans were also increasingly being christianized, which put the slave owners in a bit of a quandary. it was actually the christian authorities that reacted to this by citing biblical passages that condemned africans to slavery; there was the bit in the noah's ark story about ham, specifically. now, this was always totally ad hoc, just thrown together to prevent an uprising. in the end, it didn't work, but the long term consequences have been pretty devastating.

so, people can't understand the premise of irish slaves. they were white. what? well, they didn't have the right religion, which is the actual reason that africans were enslaved, as well.