is a metastable complex metal hydride with a predicted superconducting transition temperature as high as at ambient pressure. Following the synthesis of isomorphic, insulating at low pressure, higher-pressure studies were conducted to investigate the phase behavior and compound formation in this system. X-ray diffraction and Raman spectroscopic measurements, combined with calculations, support the formation of cubic above ca. 40 GPa. This phase may coexist with a related hexagonal hydride with an estimated composition near . Electrical transport measurements suggest that cubic is insulating, in agreement with predictions, and persists during room-temperature decompression until $\sim$ before reverting back to cubic . The experimental results support ground-state structure predictions in the Mg–Ir–H system, and the formation of two nearly identical phases with surrounding compositions may open new opportunities to access superconducting through non-equilibrium processing pathways.