Hi Col,
Killala was a register city up to about 1898 for many parishes - Killala, Kilcummin, Lackan, Rathreagh, Kilfian, etc. She could of been from any of those parishes.
I searched on the Irish TIthes (Church Tax list that was taken from 1815 - 1842) for Brown, and found a group of Browne's living in Kilfian, see here (some of the records are duplications):
https://familysearch.org/search/collection/results?count=75&query=%2Bsu…
It looks like Window Browne, Anthony, Thomas lived in Kilfian. It could be that this Thomas was her father (note that FamilySearch says that the record was from 1815, but they are wrong, it is from 1832).
https://familysearch.org/pal:/MM9.3.1/TH-1-12232-562-61?cc=1804886
Funny enough, Thomas Browne was living next to John Loughney, who was probably a brother of my great great grandfather!
Hope this helps,
John Loughney