This question comes up all the time. Here's the FAQ: http://www.reddit.com/r/AskHistorians/wiki/us_history#wiki_slavery_vs_indenture_and_.22irish_slavery.22

The short answer to your main question is that the "proclamation that Irish slavery in America was as widespread as African slavery" is totally, even laughably untrue. Something like 12.5 million Africans were transported in bondage from roughly Columbus through the 19th century. While bonded Irish labor was not unknown in the Americas, it was nowhere near as widespread as African slavery. Moreover, historians over the past two generations (roughly) have found in studying both African slaves and European indentures that it really doesn't make sense to think of Irish or other Europeans working in the Americas, even those in some sort of bonded arrangement, as "slaves." There are major categorical differences: indentures, even Irish political prisoners, had some kind of legal identity, while African slaves largely did not, and the large bulk of indentured servants received land or at least their freedom after a period of service; slaves obviously did not. This is true of convicts as well. They also did not pass their slave status on to their children, while the racialization of African slavery meant that the children of African slaves were also slaves.

As for the long post you shared with all kinds "facts," I find many of the literally unbelievable: "None" of these Irish "slaves" made it back? Not a single one? That's pretty ridiculous, since plenty of actual African slaves made it Europe (Olaudah Equiano, for example). While it is difficult to prove that something did not happen, I think you would find absolutely no support in the professional historical literature for things like the deliberate breeding of Irish women and African men to produce certain skin tones. So, overall, things like this are basically bullshit.

FYI the site that you pulled this from (and linked) is a conspiracy theory site - they also post articles which claim to tell the "truth" about 9-11, or that the US military has developed the ability to control the weather as some kind of mega-super-weapon. The book that the article is about (White Cargo) has got some attention in the popular press, but its argument is completely at odds with how historians approach this subject. It's also written by a couple of documentary makers (not historians), and it has been pretty thoroughly debunked by most of the academics who've bothered to review it:

A more serious problem is the whole business of slavery. The book is subtitled and marketed as the "forgotten history of Britain's white slaves in America". Yet as the authors admit, indentured servants were not slaves. It is true that they were dreadfully treated; indeed, Barbados planters often treated their slaves better than their servants, because the former were so vital to their economic success. The authors are right to remind us that African slavery was one form of bondage among many, rather than a unique and unprecedented condition.

All the same, it was almost always much better to be a European servant than an African slave. Not only were servants transported in better conditions, they could also hope to be free men, if they survived their term of service. Above all, they were white, which meant that they were automatically different from the West African slaves. As the servants would have pointed out, the racial codes of the American colonies were a lot more than window-dressing. Calling them slaves might be a marketing ploy, but it stretches the meaning of slavery beyond breaking point.

I'm quoting that one because it's one of the few reviews of this book by an actual historian that I can find - like many sensationalized "pop" history books, most scholars and academic journals prefer to ignore these kinds of books rather than waste their time reading or reviewing them.

More generally: As I've said elsewhere, the basic problem with works that try to retroactively assert the existence of "Irish Slavery" is that they're anachronistically conflating two different labor systems that were both common in the Caribbean and Americans from about 1600 to 1800: slavery and indentured servitude. Under the indenture system, Europeans were brought to the colonies free of charge, and in return signed on to labor for someone else for a set period of time. This was a coercive labor system, in that once someone agreed to an indenture contract they could not back out, and the "masters" of these "servants" had considerable power over them. In some cases (ie, those "Barbadosed" by Cromwell), these people were also forcibly indentured as political prisoners. But these people were the exception rather than the rule, and a lot of the estimates as to their number are likely quite over-inflated. More to the point; servants were bound by contract rather than for life, and they retained most of the other rights and privileges of legal persons. Slaves, in contrast, were treated as property, rather than persons; they were slaves for life, and there were basically no limits on masters' power over them. This was not a condition or a status that any European people (even the Irish) could have or would have been subjected to - Europeans had a powerful cultural prohibition against the enslavement of other Christians, and "slavery" as we think of it today was only possible once non-christian, racially distinct groups of people became available for sale.

As a useful counterpoint to White Cargo, I'd point people to this recent article in Past and Present, which makes it very clear that discussions of "Irish Slavery" like you see in books like White Cargo and online are way too simplistic and oversimplified, and often have an axe to grind. As an example, Block and Shaw start their article by mentioning the story of Cornelius Bryan, an Irishman who had been sent to Barbados, and was sentenced to twenty-one lashes on ‘the bare back’ in the 1650s for declaring (as he ate a plate of meat) ‘that if there was as much English Blood in the Tray as there was meat he would eat it.’ Sounds bad, right? People like Jordan and Walsh would leave it at that (with a story about someone being lashed) and the story would be quite grim. But Block and Shaw also followed the history of the Irish in the Caribbean for a much longer time, and looked beyond the English, Irish (and heavily politicized) sources that others have tended to rely on. What they found is telling. Checking in with Cornelius Bryan thirty years later, for example, we find that he wrote a will "in which he bequeathed a ‘mansion house’, twenty-two acres and ‘elevaen negroes with their increase’ to his wife Margaret and their six children." What this tells us - and what other academic works, like Donald Harman Akenson's If the Irish Ran the World: Montserrat, 1630–1730 also suggest - is that many of the Irish laborers who were sent to the Caribbean under the most grim of conditions during the seventeenth century actually did quite well in the long run, and had a social status, legal status, and set of experiences that were completely different from those of African slaves.

tl/dr: No one questions that a lot of Irish laborers were sent to the Americas during the seventeenth and eighteenth centuries, and that a lot of them were very poorly treated and had a very hard life. But calling them "slaves" (and drawing any kind of a parallel between their experiences and those of African slaves) papers over the many dramatic, important differences between what the Irish experienced in the Americas and what Africans did. It's just bad history.

Edit: Lost a big chunk of this post by editing an older version that I still had open in another tab (d'oh!). I've done my best to reconstruct it but there might be some odd transitions in there, sorry!

While the original poster's claims for the widespread nature of Irish slavery are spurious at best and diminishing of the horrors faced by enslaved Africans at worst, it is worth noting that enslaved Irish people left interesting and tantalizing cultural traces. My favorite is the role of enslaved Irish in Haiti; they brought with them the saints they worshipped and in an interesting moment of cultural fusion these deities were adopted into the emerging vodun/voodoo religion. The most prominent example is manman Brigitte, the Haitian female grave guardian, who is a red headed, green eyes woman, derived from saint Brigitte

A lot of people have commented here, but there were one or two things I found interesting which I don't think were mentioned.

As /u/agentdcf pointed out, this information is incredibly common, and usually used as some sort of rebuttal to the racist history of slavery. What these people neglect to mention when they bring it up however, is that the Irish weren't viewed as equal on any level to the English. The fact that they were 'white' accounted for nothing. They were a different ethnic group, and an inferior one. This continued on well past the age of indentured servitude.

Also, the laws surrounding indentured servitude were incredibly lax prior to 1682. It was only then that Charles II instituted a law mandating that the contracts they signed had to be witnessed by magistrates. Prior to that, there were many claims of kidnapping, ill treatment, abuse, and slavery. Historical sources don't shed much light on the kidnapping charges, as they were criminal acts and thus not recorded. But it's postulated that the claims were in some cases true(an example is of a man who returned to England and testified to Parliament that his sister had had him transported), and in others, were merely made by people who were faced with a life in Virginia far different to expectations. There were rumours as early as 1649 that anyone who signed an indenture contract was sold as a slave upon reaching Virginia.

Lastly, after 1682, the volume of African slaves notably increased. They were cheaper than indentured servants, and by this time, the English were able to ship them directly from Africa, whereas before they'd come via the Caribbean. The law that was signed by Charles regulating indentures also slackened the trade.

"One British ship even dumped 1,302..."

Is this correct that a single ship of the 1790--1800 period could hold that many people even packed as cargo?