How do I find out information about an ancestor who wasn’t an American citizen?
I agree with Maxi. My own stop is my fathers mother who came to the US alone as a serving girl to another family. I know when she came but not WHO she came over with, and have searched every ship manifest I can get my hands on and cannot find anyone with the name she supposedly had...so I am stuck. that line of inquiry is dead to me.
Maxi gave the best answer you're going to get. If you want to grasp at straws, take a DNA test from one of the companies that offers to put you in contact with distant cousins and hope for a match.
I'd suggest that you go to one of the Family History Centers (FHC) run by the Mormon/LDS church. The FHCs are located in Mormon/LDS churches all over the country. Their locations and hours are usually available online or in your local phone book. Their librarians are knowledgeable about genealogy research and should be able to tell you if there's a realistic possibility of finding more information. It may be that your ggg grandfather was baptized in a local Catholic church and his father's name is in the records there. Or it may be that you've reached a dead end. But go to your local FHC and find out what kind of research is possible there. Their services are free and the librarians are very helpful.