How to safely put mirror tiles on the ceiling?
A good coat of construction adhesive should do the trick if your plaster has good adhesion to the drywall underneath, and it flat enough for you to get a lot of contact surface. I would test this though and try to pull one down after it has cured. You will have to temporarily support the tiles with clips or something while it cures though.

Another option would be to use thinset tile adhesive and mount them just like regular glass tiles would be. You would apply thinset to the ceiling and the tiles with a notched trowel, and also pull one off after you set it to ensure you are getting near 100% coverage. It would be strong, but again, limited by the adhesion of the original plaster to the ceiling drywall.

You also might be able to use some kind of clip that is mounted using drywall anchors. This would provide the most piece of mind I imagine, especially if you are sleeping under it. Something like this tongue and groove board clip: