Pokemon GO dataminers recently found evidence of Jirachi joining the game soon. When we shared that info, there was no word on when to expect Jirachi to be available. That's now changed, as dataminers have continued poring through the game to find new info.
The latest round of info shows that Jirachi is going to make its debut at Pokemon GO Fest Chicago on June 13th to 16th, 2019, and it will only be available at that event. Attendees will have to complete a quest in order to get the Pokemon. It's likely that Jirachi will also appear at other Pokemon GO Fests lined up for Dortmund, Germany, Yokohama. If you're looking for a regular public distribution of Jirachi, it likely won't be until after those events have come and gone.