John McNamara, Boston, Ma. 1848 - 1865 ??

I've been trying to trace John McNamara husband of Catherine (family name unknown)in Boston, Mass. He shows up in the 1850 Boston census and the 1855 State census, but after that I can't find any info on him. His wife Catherine and son James A. are in the 1865 State census and she is listed as married, which leads me to assume that he was either sick in a hospital or working out of town or dead by the 1865 State census. They had six children:
John b:1836, Ireland, d:??
Martin b:1839, Ireland, d:1907 Buffalo,NY
Michael b:1841, Ireland, d:1864 Po River, Va.
Dennis b:1844, Ireland, d:1859 Boston,MA
Johanna b:1849, Boston, d:1857 Boston,MA
James b:1851, Boston, d:after 1910 Boston,MA ??

Any ideas on how to find out when John and wife Catherine died ?? Also can't find any info on son John, after 1850 Boston census. I found a few John McNamara's living in Boston in the same time frame, but can't figure out which one could be related. Or what County they came from in Ireland ?? I have both Martin's & Michael's Civil War records but do not list what part of Ireland they were born in. One odd piece of info that I do have is that Martin, Michael, James all have the middle name of Ambrose ?? Michael even named his son born in 1863 in Boston, Michael Ambrose. Anyone have an idea why they would use the same middle name. I'm assuming a grandfather or maybe their mother Catherine's maiden name ??

Thanks,
Norm Hamilton