What exactly is wrong with Irish or Italians specifically converting to Protestantism?

[𝕭𝖆𝖕𝖙𝖎𝖘𝖙𝖆 𝕸𝖎𝖓𝖔𝖑𝖆]

There used to be tons of Calvinists in Italy in the 16th and 17th century, right?

I would not really say tons but it was a fairly significant phenomenon in the 16th century... however it collapsed almost entirely by the 17th century. Either way that's probably not very relevant to the conversions of contemporary Italian Americans.