We present a study of the micro-structural changes induced in Cr-N layers by irradiation with argon ions. The layers were deposited by reactive ion sputtering on (100) Si wafers, to a thickness of 240-280 nm, at different nitrogen partial pressures and different substrate temperatures. The samples were subsequently irradiated with 120 keV Ar+, to 1x1015 and 1x1016 ions/cm2. Structural characterization was performed with Rutherford backscattering spectroscopy, x-ray diffraction analysis and transmission electron microscopy, and we also did electrical resistivity measurements on the samples. It has been found that the layers grow in the form of a polycrystalline columnar structure, with a columnar width of a few tens of nm. The layer composition, Cr2N or CrN, strongly depends on the nitrogen partial pressure during deposition. Ion irradiation induces local micro-structural changes, formation of nano-particles and defects, though the structures retain their polycrystalline nature. The induced crystalline defects yield an increase of electrical resistivity after ion irradiation.