In this work, we present a comparative study of the ion irradiation effect on the mechanical and optical properties of fullerene, amorphous carbon (a-C) and polymeric hydrogenated amorphous carbon (a-C:H) films, irradiated with N ions at 400 keV in the fluence range from 1013 to 3×1016 N cm−2. Modifications in the carbon structure, as function of the irradiation fluence, were investigated using the Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), Fourier transform infrared (FTIR), Raman spectroscopy, UV–Vis–near infrared (NIR) spectrophotometry and nanoindentation techniques. After high fluence, the three carbon samples are transformed into very similar hard (≈14 GPa) and non-hydrogenated amorphous carbon layers with very low optical gap (≈0.2 eV) and an unusual sp2-rich bonded atomic network. The mechanical properties of the irradiated films correlated with the bonding topologies of this new sp2 carbon phase are investigated through the constraint-counting model. The results show that the structural modifications and the unusual rigidity were achieved by the distortion of the sp2 carbon bond angles, giving origin to a constrained three-dimensional sp2 carbon bonded network.