Mega Jirachi is a mysterious creature that the artist has not really told us much about. We do not know if its type has changed, if it has a new ability, or how its stats have changed. So really we are all left to imagine what Mega Jirachi would be. Psychic/Fairy type, maybe?
While the lack of information is certainly daunting, Deltheor's vision of a Mega Jirachi does the Pokémon justice. Mega evolution generally makes a Pokémon stronger (though stat drops are possible) so it is only fitting that Jirachi uses its third, or "true" eye.