[dspeyer]

It is weird that this exact phrase kept getting used against the Irish and not other discriminated-against groups. Nothing for Poles, Slavs, Italians, Russians... Or at least that's what I'm finding on the LoC search. A few "No Jews Need Apply", but only a few, and those aren't job listings.

Probably not a particularly important mystery, but weird.

[skeolawn]

There are likely some historical reasons. Here's a quote from The Times from 1860:

If this [exodus] goes on, as it is likely to go on…the United States will become very Irish….So an Ireland there will still be, but on a colossal scale, and in a new world. We must gird our loins to encounter the Nemesis of seven centuries’ misgovernment. To the end of time a hundred million spread over the largest havitable area in the world, and, confronting us everywhere by sea and land, will remember that their forefathers paid tithe to the Protestant clergy, rent to absentee landlords, and a forced obedience to the laws which these had made. (The Times, quoted in The Nation, May 1860)