I asked my Mum.
She knew of only two people with the surname Devereux on the Graig; Maura and her Mum. She said that they emigrated to America. The mother would have been Mrs Devereux, anyway, and the daughter would have changed her name on marriage. She said that the father had died so she didn't really know him.
Maybe there were others she didn't know. There did tend to be extended families living close to each other.