I also heard that law was passed back in the mid 1800s to curtail the Irish street gangs, along with banning concealed weapons. I might have heard wrong though. But I bet here someone knows for sure.
You're probably right about when the law was enacted.
I was thinking about the news reports from 1999 about the last time this law was challenged, and the courts upheld the anti-mask law being used against the Klan.
http://hcs.harvard.edu/~dnd/pages/archives/2000/winter/unmasked.html
Though I think Judge Baer was right in his decision, not sure how he distinguished this case from the 1999 case.