here's another arrival you can pencil into your notes, but only because of the person's surname listed just below Bridget on the record:
6 April 1869, NYC, Bridget Moore, age 35, spinster, next line: Mary McGinness, 20, spinster
https://search.ancestry.com/cgi-bin/sse.dll?indiv=1&dbid=7488&h=6916545&tid=&pid=&usePUB=true&_phsrc=uHz28&_phstart=successSourceand here is
Pat Moore, 20, Mike Moore, 22, Bridget Moore 18, and Sarah Moore, 19 traveling together, arriving 26 Aug 1852, New York
https://search.ancestry.com/cgi-bin/sse.dll?indiv=1&dbid=7488&h=399215&tid=&pid=&usePUB=true&_phsrc=uHz28&_phstart=successSourcebut you said all the other siblings stayed in Ireland ... and no clue in the manifest as to where this group were from.
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So, I went through the first page of results at ancestry.com for Bridget Moore, b 1835, Rathfriland, arriving 1870. ancestry widens out the dates anyway. Lots were born too early or late, or arrived as children. I eliminated 9 more, found 5 maybes, but with no So Down surname as possible companion, and that left the two I posted. There are I don't know how many more pages of results. - just for Bridget.
I was hoping to find the 3 sisters traveling together. But the children usually left one at a time in all the families I have seen, so there wasn't much hope of that.
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You are looking at a problem here. Is there anyone else who might have a little more information about the three sisters, especially where in the US they settled? even a rumor would help. Were they said to have married?
Catherine's maiden name might help on the US records, if the 3 sisters lived long enough, past 1900 or so. Also siblings names might help find them based on naming patterns. Religion would help also, to identify or eliminate them based on maybe US marriage record or burial information.
Thinking out loud and rambling here, but trying to find an angle we can id these people with.