Particle shape irregularity governs grain-scale interactions that influence dune stability and surface mobility in aeolian environments, yet its effects remain poorly quantified in constitutive models. This study presents a micromechanical investigation into the constitutive behavior of aeolian dune sand (ADS) at the scale of representative volume element (RVE), with a specific focus on the fundamental role of particle irregularity. Through a series of discrete element method (DEM) simulations incorporating high-fidelity particle shapes, assemblies with varying overall regularity ( O-R) indices are systematically analyzed under direct shear test conditions, with varying packing density and normal stresses. The results reveal that as O-R decreases, both the peak and critical state shear strength increase non-linearly, demonstrating a saturation effect at high particle shape irregularity. A novel asymptotic model, phi = B - A(O-R)(n), is proposed, which provides a superior fit than the traditional power-law models by capturing this physical limit and implying an upper-bound strength for the ADS. A corresponding enhancement in dilatancy is observed, governed by a stress-dilatancy relationship with a material constant of 0.61, specific to the morphology of ADS. Micro-structural analysis shows that the evolution of fabric anisotropy and the mechanical coordination number strongly correlate with the macroscopic stress-strain response, with more irregular particles developing a more stable and anisotropic load-bearing network. A key finding is the establishment of unified scaling laws where the particle regularity index directly governs the pressure-dependence of both the critical state void ratio and coordination number. This provides a cross-scale framework linking particle irregularity to the bulk constitutive behavior, offering a micromechanical basis for the development of enhanced constitutive models for granular materials in aeolian environments.