Abstract: To investigate the sequential relationship among the hygrothermal state, moisture loss, irradiation-induced shrinkage, and mechanical response of concrete subjected to long-term gamma-ray irradiation, a two-phase mesoscale finite element model was developed in Abaqus. The concrete specimen was represented by a mortar matrix and randomly distributed spherical coarse aggregates. The mortar was treated as the irradiation-sensitive phase, whereas the aggregates were regarded as elastic inclusions that constrain mortar shrinkage. A user-defined field (USDFLD) subroutine was employed to update the accumulated gamma-ray dose, temperature, relative humidity, evaporable water content, mass loss, and damage-related state variables at the integration points. A user-defined expansion (UEXPAN) subroutine was then used to convert the dose-induced shrinkage and moisture-loss-induced drying shrinkage into equivalent eigenstrain increments in the mortar phase. In this manner, the irradiation-ageing step and subsequent compression-loading step were sequentially coupled. The model was validated against the gamma-ray irradiation tests on Con-A and Con-B concretes reported by Maruyama et al. The comparisons included specimen temperature, evaporable water content, shrinkage deformation, residual stress, irradiation-induced damage distribution, stress–strain response, compressive strength, and elastic modulus. The results show that the proposed model reproduces the periodic temperature variation, gradual decrease in evaporable water content, and development of local residual stresses and damage resulting from the restraint of mortar shrinkage by aggregates. The simulated stress–strain curves, compressive strengths, and elastic moduli are generally consistent with the experimental trends. Within the validated ranges of dose rate and irradiation duration from 4 to 32 months, the model describes the sequential transfer from prescribed environmental and moisture histories to mortar shrinkage, residual stress, irradiation-induced damage, and subsequent compressive response. However, microstructural mechanisms such as pore-structure evolution, continued hydration, and changes in calcium–silicate–hydrate structure are not explicitly incorporated. The developed mesoscale modelling framework provides a numerical tool for evaluating gamma-ray irradiation ageing and the long-term service performance of nuclear concrete structures.