1840 US Federal Census
John McElhagin, New York Ward 14, New York.
Head of family plus the Free White persons:
one male (30-39);
one female (20-29);
one female (30-39).
The three 'free whites' must have been born 1810-1820 and on the assumption that John, for whom no age is given, is a generation older he could be John McElhager born 1788 who emigrated from Ireland and ended up in Jamestown, Pennsylvania. He married and I think had children.
1900 US Federal Census
Wm C. McElhager, age 21 born Pennsylvania in about 1879.
His home in 1900 was San Isidro Huos Esijo, in the Philippines Islands.
William was a Private in the Military and Naval Forces. If I interpret the rest of this entry correctly he was in a Signal Unit.
I have no other reference to a William C. McElhager though there were a couple of Irish immigrant McElhager families in Pensylvania from the mid-nineteenth century and presumably he belongs to one of them.
1910 US Federal Census
There is a family of five in Collier, Allegheny, Pennsylvania, none of whom I have heard of before.
Addison J. McElhage, age 35, born c.1875 in Pennsylvania, whose father and mother were both also born in Pennsylvania. His wife was Anna B. (no maiden surname given), age 29, born c.1881 whose parents were both born in Pennsylvania.
They had three children:
Mary A. age 10 b.1900 Pennsylvania;
William L. age 8 b.1902 Pennsylvania;
Aleta B. age 5 b.1905 Pennsylvania.
Again, with the McElhage version of our clan name, it could be that they belong to either of the Irish immigrant families (having dropped the final 'r'). However, there may be another link for this nuclear family for there is a McElhago who is buried in Allegheny Cemetery, namely Margaret who died in 1875, and I think she relates to the Scottish Ayrshire family.
I should add that I have twice written to the Allegheny Cemetery authorities to try to get more information about Margaret's burial, the second time sending the requested $10, but on neither occasion have I received a reply.