you need to remember that the person giving the information to the census enumerator, usually verbally as many were illiterate, would only give the information they had, and if they remembered a place of birth incorrectly, the entry is then recorded incorrectly. your gg grandmother may well have been absent when the enumerator called to collect the return and a relative may have thought he or she (usually he as the head of the house was responsible for the return) was from cork, maybe they had relatives there, maybe they remembered cork being mentioned.
Any record that relies on the fallibility's of the human mind has to be accepted as often less than accurate.
Imagine when she came back from the market and her husband said, ' The census chap came and i told him you were from Armargh is that right mrs?'
and your gg grandma says, ' No you blithering idiot that's where my sistyer lives now i was born in Cork!!!' and she throws a stick at him, see, human error is a canny thing.