People are still fighting over Ireland... We fought the British for literally hundreds of years. We were fighting them before America even existed. There is still serious tension in the north, there isn't bombings any more, but they sure aren't all getting along.
I don't see how any of this has anything to do with the extreme polarization of american politics? I don't see why any of this makes it so there is an even split with no middle ground between the groups. It sounds like it should be much more diverse since you are saying it is inhomogeneous. And that is what I find so strange. America politics seems incredibly homogeneous, way more than you would expect for such a large and varied country. There is an almost perfect split down the middle and each group has very tightly defined goals which are generally the exact opposite of the other one. I find it very hard to understand how all these disparate groups over thousands of miles all ended up in one of these two groups, yet smaller countries are fragmented.