Mr MacKenzie is correct as I sing it. Fear is the noun itself. The Vocative case is the one which English dropped, where we would have the name "John" and to speak or address John, we would have said "Oh John". The Vocative for Fear is Fhir. Rules are quite standard for most things in Gaelic. Less than languages like English which is full of inconsistencies or even French where most things seem to be irregular. 8-)