Agricultural irrigation is essential for maintaining crop growth and ensuring food security. However, the potential effects of intensive agricultural irrigation and groundwater extraction-rebound cycles on surface deformation remain poorly understood. The Multi-temporal Interferometric Synthetic Aperture Radar (MT-InSAR) combined with a block adjustment method was employed to investigate surface subsidence in the irrigation areas of the Hohhot-Baotou-Ordos-Yulin (HBOY) urban agglomeration, a typical dryland in northwestern China. Vertical surface deformation in the HBOY irrigation areas was estimated by processing 1434 Sentinel-1A images acquired between early 2018 and mid-2022. The results were validated against 49 in-situ GNSS measurements, with an RMSE of 1.33 mm/year. Extensive surface subsidence was observed across the irrigation areas, and 12.64% of the region experienced subsidence rates exceeding 10 mm/year. The most significant surface subsidence occurred in the irrigated croplands of Hohhot, forming three obvious subsidence funnels. Seasonal signals in the time series of surface deformation and hydrological factors were decomposed and their intrinsic relationships were explored using continuous wavelet transform and cross wavelet transform analyses. The results revealed that surface deformation, groundwater level, precipitation, and soil moisture exhibit a dominant annual cycle. Surface deformation was positively correlated (in-phase) with groundwater level and negatively correlated (anti-phase) with the other factors. The seasonal response of surface deformation to groundwater-level changes lagged by 33 to 185 days (from the lower to upper quartile), with a median of 76 days. These findings clarify the dynamics of irrigation-induced surface deformation and provide key insights for mitigating subsidence risks and promoting sustainable groundwater management in dryland urban agglomerations.