Paul, is this your ancestor? If so, there might be an obituary in old St. Louis newspapers that mentions his Civil War service.

http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GSln=orr&GSfn=david&GSbyrel=in&GSdyrel=in&GSst=26&GScntry=4&GSob=n&GRid=52466250&

A David Orr 22 Ireland, occupation Printer, arrived New York from Liverpool 12 March 1853 on ship "Queen of the West" but I can't find him in the 1860 census either.