The requirements when completing a census was that the relationship to the householder(John) was given.But they may not have been literate and Ann could be related to either of them.
Although I think I'd go along with her being a half sister to John,maybe his mum remarried to a Mr McMahon?
Irish records are so difficult to find,I've given up on my half Irish husband's side of the tree

If only it said more than just Ireland,a place for example.
Carol