Colony irradiation is unlocked after destroying Earth.
Successful irradiation doubles the production of a random colony. Unsuccessful irradiation yields some Bacteria. One colony can be irradiated multiple times.
It is only possible to irradiate colonies that you have purchased at least once before. If every colony has been purchased at least once, then each colony has the same probability of being irradiated.
The irradiation effect is not lost when entering a Black Hole or attacking a planet.
Only one irradiation type can be researched at a time.