The handle of a utensil has the same status as the utensil itself. If one only koshered a utensil, but not the handle, and then cooked in it, the Rama (או”ח תנא:יב) rules that the food is permitted to eat after the fact. The Shulchan Aruch argues and rules that the food is forbidden. The Rama rules that one may do iruy on a handle, even if the utensil itself requires hagala or libun. This is because the beliot are transferred into the handle through the pot being heated. This means that because part of the utensil is hot, it transmits beliot to the rest of the utensil. This is comparable to absorbing beliot through iruy, since iruy uses the heat from one utensil to transfer beliot into another utensil. Therefore, one can use iruy to kosher the handle.