They came as slaves: human cargo transported on British ships bound for the Americas.
I read it in Russian and we once discussed it with my friends. First of all it was enjoyable that we were reading and discussing the same article: in the USA and in Russia. :) I'd like to notice about the picture: this is a picture of French revolution, not of Irish slavery. The author illustraits his article with the different picture.
And I'd like also to notice that slavery wasn't in Russia. We had feudalism, not slavery. Feudalism is a sort of slavery of course but not the same one. That's why Russians often means that slavery is the trouble of Europe, not of Russia.