amarriageoftrueminds | Significant aspect of Steve Rogers being Irish-Catholic:

As well as having branches of the Nazi party (ie. the German-American Bund), the Brooklyn of Steve’s day also had a big problem of antisemitism amongst the local Irish-Catholic population.

Examples:

There was a Canadian-American Catholic priest and demagogue named Charles Coughlin, who received indirect funding from the Nazi party. Up until 1939 'Father Coughlin’ had both a regular right-wing talkradio programme, and a matching newspaper (ironically called Social Justice), broadcasting pro-fascist, anti-communist, virulently antisemitic diatribes. (He was the inspiration for the character Brother Justin Crowe in the show Carnivale.)
In New York, Coughlin’s writings prompted the creation of an antisemitic organisation called the Christian Front, which held public rallies on the intersecting corners of Jewish/Irish neighborhoods in Brooklyn, with the specific intention of drumming up conflict. And just as in Nazi Germany, this indoctrination started young: there were Hitler Youth summer camps in both New York and New Jersey.
In wider pop culture, the most successful broadway play for a long time was Abie’s Irish Rose, a “schmaltzy interfaith romantic comedy" about the conflict between a Jewish family and Irish-Catholic family when a Jewish boy marries an Irish girl (so, that generation’s equivalent of Guess Who's Coming to Dinner.) It was so successful that it was mentioned in the lyrics to songs of other broadway shows, spawned a long-running series of tie-in movies, and had its own radio show.
Similarly, the musical West Side Story was originally titled ‘East Side Story’ and was about a star-crossed Jewish/Irish-Catholic couple in the Lower East Side of Manhattan (where Jack Kirby lived, and Cap too, before he was later moved to Brooklyn. Those other kids Jack Kirby grew up fighting? Probably Irish-Catholic.)
This is also why, when Jewish writer Norman Lear came to rip off UK sitcom Till Death Us Do Part to make US sitcom All in the Family, in the 1970s, his choice to play the role of the bigoted father -- even though he was inspired by his own, bigoted Jewish father -- was an Irish actor whose face “screams ‘Irish.’ (Lear kept being encouraged to use that and make the character Irish-Catholic in the show, but he refused.)
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Context:

That rise in Irish-Catholic antisemitism originated with the Fascist conflation of Jews with Bolshevism, and of Communism with the persecution of Christians.
(All this while Russian-Jewish immigrants to NYC were moving into traditionally Irish-Catholic areas, competing for jobs and housing, and their success was resented.)
This also tied into the Spanish Civil War, when Russian and Communist Front groups in America aided the Loyalist forces instead of the Fascists (the Abraham Lincoln Battalion, for example, was made up of American volunteers who fought for the Loyalists, and was 25% Jewish).
In New York, the fact that the Communist party membership was mainly Jewish was treated as proof that all Jews were Communist, (and therefore enemies of Catholics), even though the majority of Jews in New York were not Communists.
In fact, it was more the case that Jews were drawn to what they perceived as progressivism -- grounded in a history of discrimination -- and since they saw Fascism as their chief threat, they were more likely to accept Communists as allies in the fight against it (just as Catholics were more likely to ally with Fascists against Communism).
Amongst American Catholics there was also distrust of FDR's liberal New Deal policies, regarded as a wave of Communism sweeping the country, since the government’s helping of the poor post-Depression was seen as a deliberate infringement upon what was traditionally the territory of the Catholic Church. (I kid you not.)
And then, since Ireland was a neutral country, the Irish-Catholics in New York were able to continue to espouse such views during the war. Unlike them, the Germans and Italians suddenly became more circumspect (as they had during WWI), for fear of appearing to support America’s enemies, and suffering reprisals as a result.
And all this was not helped by the fact that the Pope shared this view of Communism, and collaborated with Hitler on the Reich Concordat, an authoritarian pact wherein the Vatican vowed to forbid Catholics in Fascist Germany from interfering in politics.
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All of which is to say:
It’s a BIG DEAL for Steve’s Jewish creators to make Steve Irish-Catholic.
It means that, on top of being a Nazi’s worst nightmare (disabled, blonde, blue-eyed, turned into a supersoldier and yet still antifa), and fighting Bundists, Silver Shirts and ANP members, as you’d expect, Steve often would've been fighting his own people, as well.
That takes an even greater strength of character and commitment to left wing ideals.
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