Thanks for the reply. I have further questions:
1) What is the source you've used?
2) Is there a source for where and when these units were enlisted?
3) Which of Pierson's arguments do you disagree with? That there was a consciously Unionist motive behind the mutiny? That Butler's troops provided protection and helped to cause it? (What follows in the article is pretty much derivative of these two points, I think, but if you disagree with anything else in the article, don't hesitate to comment.)
I have as yet no firm opinion on Pierson's arguments except to note that large port cities (sea and river) did contain relatively large numbers of foreign-born workers, and that immigrants tended to be Unionists. Also, Unionist support was relatively stronger in cities, aside of course from the well-known mountainous regions, than in rural areas. But these general considerations may not apply in this concrete case.