Jim,
Just a few thoughts, and input from the Ancestry tree where I saw your family. The birth order of the children is Mary, John (I left him out earlier as he was not mentioned in the Find a Grave site), James, Margaret, William, Nora, Thomas. While the Irish naming tradition was popular and used frequently in many parts of Ireland, I have seen in my own family records (all of my great grandparents emigrated to the US), that tradition did not always hold fast. If you want to apply it to your family, then James' parents would have been John & Margaret; and Margaret's parents, Mary & James. Given that those pairs are not what you are looking for, I'd say they did not keep up with tradition. Do you have the certificate to attach that was done by his eldest daughter? I find it a very strange circumstance that the two sisters got their wires crossed at his death. Yes, folks fudged their BDs or probably just did not know them in the immigration records and subsequent US census records, for a variety of reasons. Regarding emigration to Cincinnati, it was a popular place during that time for the Irish to emigrate to for a variety of job opportunities. That is where my own great grandfather emigrated to from Clare. Folks usually went to where a friend or relative was already established in the US. I looked for the immigration records for James and Margaret in 1872, but did not see any...do you have that to reference as well..ie. the ship's name? Finally, regarding the young couple separating when they arrived sounds a bit unusual. Where did Margaret go to live and support herself? Sometimes family lore is just that - lore. I have plenty of it on my tree as well!
Regards,