BALLINAMORE

[Kiltaglassan]

Isn't there an Elizabeth, born 24 Oct 1917, Ardrum?

Indeed there is. I don't know why it didn't show up during my original search 

Queenie 

https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1917/01295/1535999.pdf

Ardrum townland
https://www.townlands.ie/leitrim/carrigallen/oughteragh/ballinamore/ardrum/

irishgenealogy.ie is a free family history website run by the Irish Government. It holds all the civil registration records (post-1864) and church records for certain counties.

It also holds all Protestant & Registry office marriages from 1845.

https://www.irishgenealogy.ie/en/news/154-update-to-the-civil-records-3

[Pennines]

The 1939 register has a
Mary Keegan (surname crossed through and replaced with Anthony), single, address Napsbury Hospital, St Albans, Date of birth 7 Feb 1916, Personal  Occpn. - Mental nurse.

The date of change isn't recorded but there is a capital "M" against the entry which, I believe, indicates marriage as reason for change.

Oh my gosh I actually searched for her in 1939 and couldn't find her!