Do you know what her family called her? (eg a great aunt was registered as Ann Jane and called Annie in the family, others have been Anne and called Nancy in the family etc)
How do you know her DoB was 24/12?
Would she have named any of her children with names that could be family names? By that I mean names that are not showing up as part of the names 'belonging' to her husband.
Do you know her for religion?
Do you recall any mention of gt Aunts or Uncles
What is making you think it was Co Donegal that she came from? Family Stories? These can often contain useful information.
Have searched the military file for Frank Webb. Often military had to get permission from CO to marry. You would also see where in ireland that he was posted.
NB I see that Crisane has suggested this.