An abundance of empirical data supports the use of the Master Curve, as proposed by Wallin, Saario and Törrönen, to describe the fracture toughness transition behavior of ferritic steels, particularly the notion of a curve shape that is invariant with steel microstructure (other than lattice structure). However, nuclear surveillance programs do not always contain samples of the steel that most limits reactor operations, making direct measurement of fracture toughness impossible. This suggests that a purely empirical argument cannot define the limits of applicability of the Master Curve or validate its use for all conditions of interest. In previous papers a microstructural basis for the existence of a single “Master” fracture toughness transition curve for all ferritic steels was established and limits of applicability have begun to be explored from a theoretical viewpoint. These previous papers established that all steels with the same lattice structure and cleavage fracture mechanism should be expected to adhere to transition behavior that can be defined by a single curve shape with variations in microstructure accounting only for a shift in the transition temperature. In this paper we explore the basis for “Master Curve” validity for irradiated steels by exploring how irradiation affects the microstructure and fracture mode and using the Zerilli-Armstrong constitutive model as the basis for predictions of irradiated steel behavior.