[en] We have performed static magnetic susceptibility and ESR measurements on the X-ray-irradiated organic Mott insulator κ(BEDT-TTF)₂Cu[N(CN)₂]Cl. The Neel temperature and the spin-flop transition field decrease with increasing irradiation dose. In the ESR signal at 4 K, a paramagnetic component (∼15%) from itinerant carriers appears after irradiation in addition to the parent antiferromagnetic (∼85%) and minor paramagnetic localized spin components. We propose that domains of the itinerant carriers with a size on the ten nanometer scale are induced around the molecular defects randomly introduced by X-ray irradiation. (author)