Nanocomposite polymer electrolytes (NCPEs) offer a number of interesting applications in all electrochemical devices. In present work, thin films of proton conducting nanocomposite polymer electrolyte consisting of PVA polymer matrix dispersed with nanosized Al₂O₃ fillers (size ~ 50 nm) having H₃PO₄ as dopant are prepared by well known solution casting method. These films are irradiated with 50 MeV Li³⁺ ions having four different fluences viz. 5x10¹⁰, 10¹¹, 5x10¹¹ and 10¹² ions/cm². The structural properties of pristine and irradiated nanocomposite films are studied by XRD, DSC and FTIR spectra whereas electrical properties are characterized by AC impedance spectroscopy. Wagner’s polarization and TIC techniques are used to determine the transfarence number and ionic mobility. The structural properties shows that nanoparticles of Al₂O₃ easily placed inside the polymer matrix and hence increases its amorphisity. The electrical and dielectric properties of the films have been studied for a wide range of temperature varying from 200 to 453 K in the frequency domain of 75 kHz to 30 MHz. It is observed that temperature dependent conductivity shows Arrhenius type behavior having different slope before and after glass transition temperature. It is estimated that lithium ion irradiation causes a significant enhancement in their electrical conductivity up to a critical fluence only and thereafter it decrease. The ionic transference number for pristine and irradiated nanocomposite polymer electrolyte films has been found to be nearly unity which strongly confirms the ionic nature of the electrolyte samples.