I can’t see the marriage in the statutory records.
I searched GRONI’s records for children born to McAllister & Blair in Ballycastle and Sarah is the only one (Born 1st Feb 1873). Both parents names are recorded on the birth cert which normally indicates they were married. However you could check the certificate to see. If the word “formerly” is deleted against the mother’s name then the parents were probably not married.
You can view the original certificate on-line on the GRONI website, using the “search registrations” option:
You will need to open an account and buy some credits. It costs £2 (sterling) to a view a certificate.