I have a strong feeling that if Anne Sheehy was not my Great Grandfather's sister - she would have been his first cousin.
Do you know Anne's date of birth?
Did she stay in Ireland or immigrate?
Do you know any info about Patrick's siblings?
I think one of his siblings also got married in 1884 (if not his sibling, then a close relation - she also came from Glengar)
I do live in Australia.
I look forward to your reply.