I don't know, man. I didn't get a "Scots and Irish are more violent", so much as a "Herding cultures are more violent, the Scots and Irish (at least periphery, where the immigration came from) are herding cultures."
Not necessarily. In the 18th and 19th centuries, the Lowland periphery (where the immigration came from) were quite diverse in their agriculture. A lot of arable farming went on down in the Lowlands because the land is fertile. Though herding was very common in the Lowlands and the Highlands, it's the Highlands that are more defined by that than anywhere else. I can't speak for Ireland unfortunately.
I don't mean to sound thin skinned but I don't know if this strange sensation I'm feeling is... offense. Possibly, it shouldn't be though. Still, there's something about this whole thing that smells like that fucking awful "Glasgow's the stab capital of Europe" stereotype that gets forced on us; i.e. that we're some kind of race of wild, violent hard-men that live anarchically and have feuds and make excellent cannon fodder. We're just like any other nation out there, never mind whatever preconception people may have of us.
I'm sorry, this wasn't just an excuse for me to talk about Scottish shit again, this guy has put a lot of emphasis on it though in his description for why the South is the way that it is. Really, this guy's just being overly simplistic and applying very broad brushstrokes. I agree in principle with the initial "nation" idea, my perception is that there are very different "Americas" within the USA, but this is very heavy handed and a bit vacuous in places.