It sounds like your William McD mine are not the same.
My William J. McD was born in Chester County PA 1855.From there he moved with his Father JOHN and Mother NANCY BRADLEY and older brother JOHN P.(also born in Chester) to Clinton County Iowa.John and Nancy had 4 more children in Clinton County.
William J. McD married Margaret DUNLAVEY/DUNLAVY in 1880 in Clinton County.
I've been able to follow my McDs through Iowa census in Clinton County and can pretty much track them from their arrival in Clinton County Iowa to the present.It's the time prior to that that I am trying to research.
I know that John McDevitt came from County Derry Ireland in 1848 to Nova Scotia then Pennsylvania and Nancy Bradley came from County Donegal Ireland (date unknown) and they left PA aprox 1855 and settled in CLINTON COUNTY Iowa.
Perhaps your McDevitts are cousins to my McDevitts since they seem to have followed a similar migratory pattern.