Gao et al. (2021b) attempted for the first time to use Bragg reflection to mitigate the harbor resonance induced by regular long waves and bi-chromatic short wave groups, revealing its remarkable effectiveness. However, in actual coastal/offshore areas, waves usually exist in the form of irregular wave groups. This article considers the more practical irregular wave groups as the incident waves that excite harbor resonance. The harbor resonance occurring inside an elongated harbor triggered by irregular wave groups and its interactions with Bragg reflection are simulated using a Boussinesq-type model, FUNWAVE 2.0. This article reveals for the first time the capabilities of Bragg resonant reflection to effectively alleviate both the main resonant mode and the whole resonance of the harbor triggered by irregular wave groups. The influences of the geometric parameters of bars (including the bar amplitude and the bar number), the spectral bandwidth parameter of incident irregular wave groups, and the main resonant mode on the following four key parameters are further studied comprehensively: (1) the optimal mitigation effect for the main resonant mode, and (2) its corresponding optimal bar wavelength, (3) the optimal mitigation effect for the whole resonance of the harbor, and (4) its corresponding optimal bar wavelength.