Thanks for the replies all. My only evidence, apart from her photo, is the birth certificate for my father, her son. It's for 1916, lists father as John Aitken, her as Susan Aitken, formerly Oliver, with the address of 14 Jaffa St. Now as to where I got that birth date from (after all it makes her 16-17 when baby born) I can't recall, but I must have got it from somewhere as I don't put dates down willy nilly! I suppose then I can assume it may not be certain. The idea that she'd been married before, funnily enough, was also mentioned by my stepmother who may have got it from dad. It sounds like, between you, you've found her for me! Which is pretty amazing as I just hoped someone might refer me to an Irish site I might not be aware of! Now I find myself wondering if she's the mother of the boy, James.
Anyway, thanks so much guys, you've been brilliant!
Catherine