For the purpose of referring to the ethnic group, I won't say English, but Anglo-Saxon. I know it's a bit confusing, since the USA fought against the Crown (even though I don't think "The Patriot" was anti-English; since both of the people fighting each other were Anglo-Saxons; "Braveheart" was racist because its pits the Scottish against Anglo-Saxons), so usually they won't refer to themselves as English but American. Ever wonder why "Anglo-Saxon" is a symbolism for racists. When people talk demonize racists, it's always by calling the racists "Anglo-Saxons." Celts are not associated with racism. Anyways, it was the Anglo-Saxons who brought the slaves to this country. Even the ones who were against slavery had no problem finding others ways of oppression. It was them that were puritanical in many horrible ways. In many cases the Scandinavian element in England made it even worse. After all, while the Normans were far from perfect, they did end the barbaric practice of slavery started by Anglo-Saxon invaders in 400, and later Danish invaders some 200 years before the Normans settled in England. But aside from the real Anglo-Saxons, the Anglo-Saxon ethnic group was the one predominantly making the Jim Crow laws (as well as Scandinavians to a certain extent).
No, the Scots-Irish were not more to blame for slavery, because there were far more Anglo-Saxons leading the way into slavery in the 18th and 19th centuries. The Anglo-Saxons, especially the ones in the south, made it clear that they did not see the problem with slavery. The Scots were less responsible because they were just trying to survive.
Anglo-Saxons are what many people in the United States would be considered to be. I would have to say the Anglo-Saxons are more responsible for slavery because many people in America kept slaves. This is just the way it was. Although it is not true today, more Anglo-Saxons in the past kept slaves than the Scots-Irish.