A likely explanation is the strong group ethos that encouraged Irish to always work together, and resist individualistic attempts to break away. (The slogan tells them that trying to make it in the Yankee world is impossible anyway.) No other European Catholic group seems to have shared that chip on the shoulder (not the Germans or Italians—not even anti-Irish groups such as the French Canadians). Historians agree the political hostility against the Irish Democrats in the Civil War Era was real enough. Critics complained that the Irish had poor morals and a weak work ethic (and hence low status).
Much more serious was the allegation that they were politically corrupt and priest-controlled, and therefore violated true republican values. The Irish could shoot back that The Enemy did not practice equal rights. The Irish community used the allegation of job discrimination on the part of the Other to reinforce political solidarity among (male) voters, which in any case was very high indeed—probably he highest for any political group in American history before the 1960s.
It is easy to identify job discrimination in the 19th century against blacks and Chinese (the latter indeed led by the Irish in California). Discrimination against the Irish was invisible to the non-Irish.
That is perhaps why this urban legend did not die out naturally. Benign Protestant factory owners could not soften the tensions by removing signs that never existed. When Protestants denied NINA that perhaps just reinforced the Irish sense of conspiracy against them (even today people who deny NINA are suspected of prejudice.) The slogan served both to explain their poverty and to identify a villain against whom it was all right retaliate on sight—a donnybrook for the foes of St. Patrick.
The myth justified bullying strangers and helped sour relations between Irish and everyone else. The sense of victimhood perhaps blinded some Irish to the discrimination suffered by other groups.
Perhaps the slogan has reemerged in recent years as the Irish feel the political need to be bona-fide victims. The Potato Famine of course had all the ingredients to make them victims,but it will not do to have the villains overseas: there must be American villains.
If we conclude the Irish were systematically deluding themselves over a period of a century or more about their primary symbol of job discrimination, the next question to ask is, was it all imaginary or was there a real basis for the grievances about the economic hostility of Protestants to Irish aspirations? Historians need to be critical. Because a group truly believes it was a victim, does not make it so. On the other hand, the Irish chip-on-the-shoulder attitude may have generated a high level of group solidarity in both politics and the job market, which could have had a significant impact on the on the occupational experience of the Irish.
How successful were the Irish in the job market? Observers noticed that the Irish tended to work in equalitarian collective situations, such as labor gangs, longshoremen crews, construction crews, or with strong labor unions, usually in units dominated numerically and politically by Irishmen. Wage rates were often heavily influenced by collective activity, such as boycotts, strikes and union contracts, or by the political pressures that could be exerted on behalf of employees in government jobs, or working for contractors holding city contracts, or for regulated utilities such as street railways and subways.
The first arrivals formed all-Irish work crews for construction companies in the building of railroads in the 1830s.
Sometimes the Irish managed to monopolize a specific labor market sector—they comprised 95% of the canal workers by 1840, and 95% of the New York City longshoremen by 1900.
The monopoly of course facilitated group action, and once a crossing point was reached it was possible to exclude virtually all Others.
Solidarity (with or without formal union organization) made for excellent bargaining power, augmented as needed by the use of intimidation, strikes, arson, terrorism and destructive violence to settle any grievances they may have had with their employers, not to mention internal feuds linked to historic feuds back in Ireland. Direct evidence that employers did not want Irish workers is absent. By the early 20th century major corporations had personnel offices and written procedures.
If the Irish had a reputation for being unsatisfactory, the personnel managers never commented upon it. Job discrimination against blacks and Asians continued, and was quite visible in the corporate records and the media. Discrimination against newer immigrant groups can be identified as late as 1941 (when it was banned for government contract holders). No trace of anti-Irish hostility has turned up in the corporate records of the literature of personnel management. Can we prove there was no job discrimination against the Irish? Zero is too hard to "prove"—though no historian has found any evidence of any actual discrimination by any business or factory.