I take issue with your claim that Irish were "not considered white." Such an argument only makes sense given a prior assumption that "white" only means a vague definition of acceptability in society. Under that stand-in definition (and frankly a modern revisionist definition), the Irish immigrants who were considered outsiders would be tautologically "not white," but that's not how people at the time thought of it.

As evidence I present the Naturalization Act of 1790 [0], which restricted US citizenship thusly:

>That any alien, being a free white person,Alien whites may become citizens, and how. who shall have resided within the limits and under the jurisdiction of the United States for the term of two years, may be admitted to become a citizen thereof

Note that this was far before the period of "mass immigration to the US." The "free white" requirement remained in US law until the mid-19th century. I've never known of any case in which an Irishman was denied US citizenship during that time period on account on "not being white" (I would be happy to be proven wrong here). There were several signers of the Declaration of Independence that had Irish ancestry! [1] An outsider group, yes, and one that took time to be integrated as a full part of the in-group, but the idea of people of the time looking at an Irishman and saying "he's not white," would not have made sense.

You're right that the concept of "white" as an identity group is relatively recent (on the order of several centuries) and was largely born in the American colonies as a way to distinguish an in-group from an out-group (mainly, colonists from various areas of Europe wanted something to distinguish themselves from the Indian tribes and black African slaves). But that's the way of all these categories ("black" can be similarly deconstructed as a recent American ethnogenesis).

[0] https://en.wikipedia.org/wiki/Naturalization_Act_of_1790

[1] http://thewildgeese.irish/profiles/blogs/the-irish-american-...