Breandán wrote:
An Cionnfhaolach wrote:
Should that not be "a Óinsigh", no?
Óinseach is a feminine noun so I am thinking "no". I expect it to function like
gealach, vocative
a ghealach, hence
a óinseach.
Clarification by more advanced speakers would be welcome.

Well, I don't know if I'm more advanced, but the vocative would be
A Óinseach in most dialects, as it is a feminine noun.
However in Munster there is a tendency to inflect feminine words as if they were masculine in the vocative. So in
Munster you have
A Óinsigh.
Yes, I saw that comments in your Munster Case System thread (another interesting and informative thread
(I was about to add a comment here but another matter intervened - I've since split that off into its own thread.)