First of all, I certainly thank all of you for your terrific responses!
I'll try to give you as much information as I have (which isn't a whole lot, unfortunately) for all of you wonderful sleuths out there, as this is surely one big puzzle.
We don't know Peter J. Casey's father's name. That's what we are now looking for. Peter was born in Ireland in Jan. 1841 (year could be slightly off) and died in Louisville, Ky. 10 Jan, 1910. Records that gave information regarding a deceased person's parents didn't begin in Louisville until 1911!
As per the 1900 census, he came to US in 1856. His Declaration of Intent, which we DON'T have, was on 20 Oct., 1858 and he naturalized on 20 Oct., 1860 in Hamilton County, Ohio. We DO have his naturalization papers but the only information on them is that he was born in Ireland and that a Patrick Casey was his witness. Hamilton County Courthouse had a fire that destroyed all of the naturalization papers in the 1880's, so that avenue is closed. Sadly, we have no idea if Peter had siblings, if he came here alone or with family. The family just never talked about it.
I can't find Peter Casey in the 1860 census in Hamilton County or Jefferson County (Louisville). I DID find a Peter Casey, age 26, John Casey, age 32, and Joseph McCaran, age 32, all together with many other Irish laborers in St. Louis in 1860.
We finally find him in the 1870 census living in Louisville with his new wife, Elizabeth (married in Dec. 1869 as per census) and a Mary Casey. Here's where it really gets interesting. My great-grandmother Elizabeth's maiden name was also Casey! Her parents were Edward Casey and Catherine Ryan, as per her death certificate. So, Mary Casey could have been either Peter OR Elizabeth's sister. I've tried to find Mary in later censuses and death records but just ran into false leads.
Patrick McCarran's involvement is also interesting. We have notes from our late aunt, Peter's youngest grandchild, that only says that Patrick McCarron was Peter's cousin. Interestingly, this aunt had Patrick's Petition papers and we learned that he came to the US in 1847 via New Orleans. I've been able to find him on every Louisville census starting with 1860, and I think he must have had brothers, Robert and Daniel, who were in Louisville during the 1850 census. I'm unable to find either of them after that, even in death records.
I was able to find out a lot about Patrick McCarron and his wife, Mary. They married in 1872 and never had children. We even discovered through HER death certificate that she was living with my grandmother and grandfather (Peter's son) at the time of her death, along with Elizabeth Casey, who was also a widow by then. The only other info from that is that Mary's father's name was John Ryan. We are thinking that possibly Mary and Elizabeth were also cousins, as well as Patrick and Peter.
Patrick pre-deceased Mary, but she was the informant on his death certificate and just put "don't know" for the names of his parents.
My grandmother was the informant for Mary McCarron's death certificate and interestingly, she KNEW that John Ryan was the name of Mary's father, but didn't know the mother's name. That's why we believe there was definitely a closeness between the McCarron's, Caseys and Ryans that would suggest they knew each other well in Ireland, at least the Caseys and McCarrons.
My aunt used to tell us she thought the Caseys were from Cork County, but only knew that the family owned a mill that went bust after the great famine.
Now, I've tried to search New Orleans passenger lists for Peter Casey, but I haven't been able to access those from 1856 - it says they're still unavailable. I found Pat McCarron on the Niagra in 1847 from Liverpool and he was 16. No other McCarrons were on the ship.
I have a feeling Peter Casey lied about his age as he went through the 2-step naturalization process, meaning he was OVER the age of 18 when he came to the US. If he were born in 1841 or so, he would have only been 15 or 16 when he came and by law he could have just done the 1-step process at age 21. Also, Peter SHOULD have been 21 at naturalization, and according to the information we have, he was at most 19 at the time.
I went through the surname search on Griffith's Valuation and did find that there were no McCarrons in the south of Ireland at that time, but there were quite a few Casey's in the north. The most promising counties were Monaghan and Leitrim. Do you think I am correct in assuming that they must have lived near each other?
So that's my sad story. Sad because so little information about a life-changing event was passed down, and sad because I'm afraid I'll never be able to learn the story.
Anyway, if you were able to get through all of this, thank you for your patience. I certainly appreciate any suggestions you may have.
Joyce