Good morning Sinann.
I gather from Aghadowey's message that this John is son of Margaret Alexander and John McMahon Snr and would be sibling to Margaret Agnes McMahon/Walsh. Why one goes to US and the other to Australia would be another interesting story.
According to John Jnr's death certificate from the US his birth would be around 1848 and Margaret Agnes is around 1852 according to her shipping record, as she was 27 when she came to Australia in 1879.
Cheers Trishmcc
No, I said that John born c1848 Ireland had parents called John McMahon and Margaret Alexander.
I didn't find him in U.S. census records yet (you probably have no idea how many John McMahons born in Ireland around that time are in N.Y.C.!) but this is likely connected (perhaps John died at that address but it wasn't where he previously lived).
1910 census-
126 Bradhurst Ave.James McMahon, 31, born NY, parents b.Ireland, married 8 years, barkeeper- saloon
wife Margaret McMahon, 29, born Ireland, 4 children/4 living
son John McMahon, 6, born NY
son James McMahon Jr., 4, born NY
daughter Hannah McMahon, 2, born NY
daughter Mary V. McMahon, 6 months, born NY
sister Elizabeth McMahon, 28, born NY, married, 1 child/1 living [wonder if this is actually James' sister-in-law]
nephew John McMahon, 3, born NY