Entry and ExitEdit
The only known way to enter the Mirror World is to be pulled in by a Reflecto Luponis. The only way out is to have the Reflecto Luponis which pulled you in destroyed within 24 hours. If it is over 24 hours, you are trapped forever.
As the victims trapped in the Mirror World appear in mirrors floating, it is possible the Mirror World has no gravity. According to Kirby, the Mirror World also only has two dimensions; (Up/Down and Right/Left presumably).