BTW: Many native-born Americans did not consider the Irish to be "white" in the 19th century. They were considered a separate race with their own set of physical characteristics, hence the 19th century caricatures with pug noses, etc.

I have heard that jig as a perjorative has a different derivation than jigaboo. Jig was the same as a dance such an Irish jig. This came about in the 1870s and 80s in America and was quite popular. It was called "jig piano" because people danced jigs to it. IOW, it was a very early form of rocknroll where people cut loose and let it all hang out. Not stately ballroom waltzing but far more sensual and "scandalous." Since blacks were the primary players of jig piano and its primary fans, the word jig began being applied to blacks in general (although it had a not insignificant white fan base also).

That sounds like a naive bowdlerization of history to me. I'd like to see some documentation, since "sensuous" dancing was (still is) virtually synonymous with "black".