I first read someone saying something like that here on Free Republic about 15 years ago. It was pretty blunt, and sort of rubbed me the wrong way (having been raised Catholic), so I went and investigated the claim. Over many years.
And it turns out that it's correct. It's interesting, if you take "Votes GOP" as a proxy for "Is Conservative" - then you can measure that. Voting GOP is an indicator that perhaps the person understands the intent of the Founders of the Republic. (So is support for an armed citizenry in my experience). What you discover, if you go look at voting patterns, is that you see that the GOP (and therefore the real American Republic) really is a WASP program, and not too many other groups have signed on to support it, even those who have done very well in America.
I believe Trump carried the non-Hispanic Catholic vote in the last election, but that was a first. Maybe, about 100 years after the high water of Irish and Italian immigration these groups are turning into Constitution supporting Conservatives. That's 5 generations, though, and it's still a lot lower level than people who self-identify as "Evangelical".
I See my posts #20 and #21.
White Catholics vote GOP at about the same rate as White Protestants, in 2016, and more generally. Hard to break out the different ethnic groups among either Catholics or Protestants. For Catholics you have your Irish, your Italians, your Polish - I have only seen anecdotal evidence of how these groups vote compared to each other.
For Protestants you have your WASPS, your Germans, but also your Swedes and other Scandinavians - who are by legend notoriously leftist.
You also have your Scots-Irish, who are notoriously notorious.