I think that I can help a bit here. First, on that 1860 US Census, look three families down the page after Dennis & Catherine Hassan, and you will find the family of James and Margaret Hassen with their children Annie, Mary, James, and Catherine. I would think that this second family is a set of relatives. Also, there is a naturalization index card out there on the Ancestry site for a Dennis Hasson dated 12 Nov 1857 placing his address at 201 West 15th Street in NYC, which was witnessed by a James Hasson of 4 Horatio Street NYC. I'm guessing that these are the same people as on the 1860 Census. 1857 is also the birth year of Dennis's first child, and so I am guessing that he petitioned for naturalization at this time due to his marriage and/or immigration. Dennis and Catherine are also out there on the 1880 US Census living at 405 West 18th Street in NYC with children Mary and John. I didn't notice a listing for them on the 1870 US Census.
The New York Times newspaper has a listing for a Dennis Hasson who died on 7 May 1898 aged about 52 years (I also stumbled upon this on the Ancestry website- there is no detailed obituary). His residence at the time was 209 West 20th Street. Although there is an age difference, I suspect that this is your guy. An 1898 NYC directory notes an Anne M. and Elizabeth Hassan living at the same address at this Dennis who died. Since neither is noted as a widow, this would indicate that they were likely either the children or sisters of Dennis. Catherine possibly died on 10 Oct 1891 in NYC aged about 57 years. You can order both death certificates from the NYC Municipal Archives, if desired. Dennis is Manhattan death certificate #12611 and Catherine is Manhattan death certificate #34958. You might want to poke around a bit more and just confirm that neither one appears on the 1900 or 1910 US Census in NYC just to be sure. The 1905 New York State Census should also be checked, which is available on the LDS website. I'd suggest checking Hassan/Hassen/Hasson as the spellings are different in all the records I've come across.
