I have just read your message,I see it was written a good few years ago and you may well have all the information you need by now.
I wonder if we share the same great great grandfather?
My great great grandfather is also called James O’Brien and he married Mary O’Leary.
I have just found a possible death for Mary in 1900 in Manhattan.
I don’t know how many children they had but Ellen is my great grandmother,she was born in 1849 and she died in Liverpool in 1928.
I look forward to hearing from you