The 1860s when the Irish in NYC refused to fight for their new country, killing over 100 people and ethnically cleansing most of Manhattan? Or a few years later when two different subgroups of Irish killed another 100 of each other in the same city? These questions re-litigating history are immaterial to the fact that there is always some point at which putting groups next to each other will cause conflict, modulated by things like population density, culture, religion, tribalism vs. individualism, prosperity, etc.