The influence of 120 MeV Ag ion irradiation on the structural and electrical properties of HfO2 (3 nm)/SiON (1 nm)/Si (n-type), grown by atomic layer deposition, has been investigated. X-ray Reflectivity and X-ray Photoelectron Spectroscopy measurements suggested the formation of a mixed interlayer of HfSiON above a critical fluence of 1 x 10(12) ions/cm(2). The observed irradiation induced changes in the leakage current have been attributed to the defects and structural changes caused by ion irradiation. The influence of various quantum tunneling mechanisms on the leakage current has been investigated as a function of ion fluence. The structural changes, defects dynamics and the consequent effects on leakage current have been explained within the framework of ion induced annealing, creation of defects and intermixing effects.