No worries, I actually only remembered this thread a couple of days ago and checked for the first time in a while and now an email pops up!
Quite an easy one to miss with such a common name. Unfortunately, I still haven't done much work at all on the O'Brien side of this line or much verification as I am focused on some DNA mysteries on another kit.
Mary Gardiner (O'Brien)'s parents are listed as Cornelius and Elizabeth in the Australian Death Index Reg #24201 1929 Katoomba. There is the birth of a Mary O'Brien registered in Sydney NSW (#1172/1863) with parents Cornelius and Elizabeth (on NSW BDM website, don't have copy).
I have DNA matches on Ancestry to another Cornelius Michael O'Brien descendent down the Jakins line as well as on his brother's Edward O'Brien's line via Eva O'Brien as well as another on their brother Joseph's line (your line) via Nancy Leith O'Brien. I have 3 DNA matches through descendants of the siblings of Frances Elizabeth Chislett.
Mary Gardiner's (O'Brien) death notice:
https://trove.nla.gov.au/newspaper/article/16610612 (as far as I know, Eileen refers to her daughter-in-law, Eileen Bieber, wife of Frederick Scott Gardiner (1893-1966) (my line), not another daughter).
Funeral notice:
https://trove.nla.gov.au/newspaper/article/239310895Unfortunately, 2 of Mary (O'Brien) Gardiner's children died by suicide.
Louisa Catherine Bright (Gardiner) 1888-1908:
https://trove.nla.gov.au/newspaper/article/229929057?searchTerm=louisa%20catherine%20bright%20glebeHenry Francis 'Harry' Gardiner (1905-1930):
https://trove.nla.gov.au/newspaper/article/16737039?searchTerm=henry%20gardiner%20katoomba (distraught after losing his mother - his father and sister had already passed)
Mary O'Brien's only other child Frederick Scott Gardiner, my great grandfather, worked at a number of Sydney newspapers including The Labor Daily, The Sun, The World, Fairfax/Sydney Morning Herald. I've been tagging a few of his articles and mentions on Trove:
https://trove.nla.gov.au/search?l-publictag=Frederick%20Scott%20Gardiner