Mary O'Grady

My great grandmother Mary O'Grady was born on May 14th 1870 at Salterhill near Slamannan in Stirlingshire. It took me some time to find the certificate as her father , Thomas O'Grady, changed the family name to Geddes.  My grandmother   had always referred to that branch as being Geddes. The census returns show the family as Geddes in 1881 and 1891. I am still looking for clues as to why Thomas  changed the  name. It is likely to be one of two reasons; to get rid of the stigma of an Irish name, or to revert to a name associated with his mother rather than his father - he was illegitimate.