Fridge Logic: If Darby loses his last wish, shouldn't Katie die?
The generally accepted explaination is that King Brian, as a gesture of friendship to Darby, takes Katie's place in the Cóiste-bodhar since, as Brian had earlier stated, it can't return empty, and Brian himself had told Darby that Katie was all right. Since leprechauns are immortal, Brian wouldn't stay dead.
It was only stated that the coach couldn't leave empty, not that the occupant had to remain in it for the duration of the trip.
