It just depends on the point of view of view. Radiation is the energy emitted by a source. Irradiation is the process of exposing an object to radiation.

Radiation is not a controlled, natural process,

Irradiation is a process controlled by you.

But in general radiation and irradiation are processes when particles come out from some source.

Radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. So, photons, electrons, protons, ions, neutrons, and so on, which have energy, are considered radiation. Whereas irradiating/irradiation is the process of exposing a body/model to that radiation.

By grammar of English language "Radiation" is a noun which scientifically means energy in the form of of wave or particle like; photon, electron, neutron ..... but " Irradiation" is a verb which is the action of exposing materials or targets by radiation.

The energy particles from a source either direct or indirect, ionizing or non-ionizing. While irradiation is the activities involved the practical uses of radiation such as x-ray diagnosis of a particular body region