firínne, what you say is true. A great grandfather of mine came and signed in the Union Army. Any Irish he knew would be quickly superceded. He married an American of English stock. Their daughter, my grandmother, learned no Irish. I think this was true in many cases. Men came over, went west, married, and the children were raised in an American environment.
Irish that remained in cities like Boston or Chicago would have Irish neighborhoods for a generation, but except for names, their descendents rapidly lost their Irish roots.