Board Thread:General Discussion/@comment-25896389-20160402095124/@comment-24674051-20160517034722

while there are some that would like to see Mary Poppins as either a villain or a hero, neither will happen. Disney does not, to my knowledge own the rights to P.L.Travers works, and She explicitly forbid Disney from making further adaptations of the Mary Poppins stories after her experience with the first film.

So the chances of Once getting the rights to Mary Poppins are None and None.