Ellen was Brannan to her maiden name, then married Patrick Cassidy, who was killed WW1.
Ellen Cassidy (nee Brannan) then, it seems from your finds, married Peter Stuart.
When Ellen Stuart, previously Cassidy (nee Brannan), then died in 1970, she would have been indexed under all three surnames:
Ellen Brannan other names Stuart. Cassidy.
Ellen Cassidy other surnames Brannan. Stuart.
Ellen Stuart other surnames Brannan. Cassidy.
So, no need to find a marriage of an Ellen Stuart to a Brannan.