All "Irradiated" legendary weapons deal radiation poisoning in addition to their normal damage.
Radiation poisoning (from the wiki) functions exactly like environmental radiation: each 1 point of radiation poisoning reduces max health by .1% (so 50 points of rad poisoning reduces max health by 5%). This gets reflected as actual damage, even if a character is at full health. Moreover, since this directly affects maximum health, this is damage that can't be healed: even legendary enemy mutations or the "resethealth" console command will restore health only up to any limits from radiation poisoning.