We present new results on the ion-assisted amorphous to polycrystal transition in silicon. The grain growth velocity vg and the nucleation rate r exhibit quite different behaviours: i) ion irradiation enhances r by a factor which is several orders of magnitude larger than that observed for vg; ii) irradiation with lighter ions produces a decrease of vg and a strong increase of r; iii) the increase of dose rate produces a decrease of both vg and r, but is particulary severe for r. These results have been explained by assuming that ion irradiation produces three fundamental effects: i) generation of long living defects which increase the free energy of the amorphous phase to the value of the fully unrelaxed amorphous silicon, causing the decrease of the thermodynamic barrier to nucleation; ii) generation of defects promoting the transition kinetics at concentrations well above the thermal equilibrium value; iii) prompt amorphization of a small volume at the surface of each crystalline grain.