Surface engineering and electronic modification are the two primary elements for improving the catalytic properties of electrode materials. We significantly improved the catalytic performances of interfacial-engineered Ir nanocluster-containing cobalt phosphide/phosphate heterostructured nanowires on nickel foam (Ir-Co₂P/Co₂P₂O₇ NWs/NF) in water splitting. The optimized Ir nanocluster content of 0.05 wt% resulted in a highly efficient catalytic performance, significantly outperforming commercial IrO₂ and Pt-C catalysts. The Ir_0.05-Co₂P/Co₂P₂O₇ NW bifunctional electrocatalyst displayed cell potentials of 1.52, 1.60, 1.62, and 1.67 V in 1.0 M KOH and mimic, simulated, and natural seawater, respectively, at 10 mA cm⁻². DFT analysis confirmed that the heterostructure enrichment in the z-oriented d-orbital facilitates strong electronic interactions between the adsorbates and surfaces. The keys were the simultaneous generation of metal phosphide/phosphate via activation of the P site on the support. Furthermore, the electrocatalyst exhibited a solar-to-hydrogen efficiency of 22.4% in solar energy-aided water splitting, indicating that it is a viable, inexpensive candidate for use in water splitting.