I thought i had tested this already but seems i didnt. You can stack many vessels with irradiate, run into enemy base and literally insta-kill stuff because of the irradiate stack :D
This seriously makes irradiate on ultralisks even more superior as 2 ultras stacked with irra actually kills both ultras.
This will be my new 3v3 trick and im going at it right now.. Mass vessel with irra only xD
The laughs..
Edit: Ive always always always thought it didnt stack so ive always spread em out when trying to do dmg with it. I.E when trying to kill drones ive spread em out through the patches to hit as much drones with dmg as possible. Been doing it wrong the whole time, unless this was recently changed that is.