william ralston and sarah gilbrath

I have found william ralston - whom I believe to be my ggggrandfather - on the igi index married to sarah gilbrath (this may be a corruption of Galbraith) and the listing of their children born in armagh.there is no listing for John Ralston, my gggrandfather.Does anyone know of this family and has he just been missed on the pedigree or has anyone information on william and sarah ralston.thanks