A solid under irradiation is a far-from equilibrium system, and therefore phase equilibria in such a system cannot be assessed from equilibrium thermodynamics. Starting from a kinetic description which incorporates the various processes responsible for atomic diffusion (e.g. thermally activated jumps, replacement sequences or displacement cascades), the various possible steady-states can be identified analytically or numerically, as well as their kinetic evolution on varying the control parameters of the system (e.g. temperature, average composition, irradiation flux, cascade density ...). Furthermore, from stochastic versions of the kinetic model, potentials governing the stationary probability distribution of states can be derived, allowing to build dynamical equilibrium phase diagrams. Illustrating the above approach on the A2-B2 order-disorder transition, we have identified irradiation-induced two-phase state, cascade size and density effects on phase stability. By incorporating point defects, such description is well suited to study irradiation-induced segregation at sinks in concentrated alloys.