The simulation study of mesoscale scintillation-producing irregularities obtained with the 3-D nonlinear model for the structuring by the gradient-drift instability (GDI) and the velocity-shear-driven instability in high-latitude plasma patches is presented. We have identified four groups of mesoscale structures based on the structuring mechanisms. The amplitudes of the scintillation-producing irregularities have been described with a normalized irregularity index which is proportional to the scintillation index S4 and accounts for the greatest variability in ionospheric scintillation. We have demonstrated that the dominant mechanism which can lead to intense scintillation is related to the patch structuring by the primary GDI and high plasma density in a patch. In the velocity shear simulations, the values of the irregularity index are found to be weaker than those produced in the simulations with no shear. The relations between the magnitudes of the simulated electric field and density irregularities have been derived for each category.