Hi Dundee,
I am looking for her birth, no success, so had a look for baptism, considering the Marriage in RC Cathedral in Aberdeen, but have not found either - and yes, - have spent quite a few credits looking at the actual records.. They were living in Aberdeen in 1914, at same address in 1911 census - already with children. (Did not find in 1901). Having said that, you have made me really look at it again, and 1911 census states that she was born in "Rosshire, Achmore" .... so that would make me think that the birth I found in "Achmore, Lochs, on 4/12/1881 - Johanna McKay, illegitimate - to Margaret McKay" is the most likely. Which means that the two pieces of information which you would normally expect to be most reliable - being "primary" - info given by herself i.e. marriage /parents names)... and precise date of birth given when she herself applied for War Pension are both wrong! She was 9 months older than stated.. and yes, she seems to have invented a fathers name and changed her mothers surname!! for the marriage? Witnesses at marriage were unknown/unfamiliar names.
Knowing how strict RC rules were, they possibly had to be fairly inventive to bypass them, and be married, (though why they choose the church?) before he went off to war, - just in case he died, so she would get pension, which is exactly what happened.
I will try again to find previous census - though there are a considerable number of "McKays" to check out. Thanks for your comments... made me look closer at 1911 census... But am coming to conclusion that Johanna certainly was not one for precision in important details lol - and that I have already got her birth!