Intelligent reflecting surfaces (IRSs) have emerged as a revolutionary wireless networking paradigm for dynamically controlling propagation environments. This paper provides a comprehensive investigation of IRS-assisted interference mitigation for mmWave MIMO communication systems. A practical 3D clustered statistical spatial channel model is adopted for accurate analysis. Adaptive passive beamforming is achieved by formulating and solving an optimization problem to configure the phase shift settings of 100 IRS elements for maximal received signal-to-interference-plus-noise ratio (SINR). Detailed MATLAB simulations demonstrate immense performance gains from deploying an IRS – radiation pattern plots exhibit a directed beamforming gain enhancement of 13.5 dB while bit error rate (BER) curves showcase 5 times improved reliability across multiple signal-to-noise ratio (SNR) regimes for QPSK and 64-QAM modulation schemes. The results conclusively validate the effectiveness of IRS-based paradigms for dynamic and efficient interference management in emerging 6G heterogeneous networks.