Chain-like Co mesospheres were employed as sacrificial templates to synthesize low-Ir-content CoIr catalysts through a galvanic replacement reaction (GRR) with Ir precursors. The synthesized CoIr catalysts exhibited hollow and porous architectures, while SEM, TEM, EDX, and XPS analyses confirmed the formation of CoIr core-shell structures with successfully incorporated Ir species. The electrocatalytic oxygen evolution reaction (OER) performance was evaluated in 1.0 M NaOH electrolyte. Among the synthesized catalysts, CoIr-(10), prepared using a 1.0 mM Ir precursor solution, exhibited overpotential and Tafel slope values comparable to those of commercial Ir-20/C, demonstrating excellent OER activity. Notably, the optimized CoIr catalyst maintained commercial-level electrochemical performance despite containing only ~3 at.% Ir. The enhanced OER activity is attributed to the high surface area arising from the hollow porous structure and the synergistic interfacial interaction between Co and Ir species. These findings demonstrate that the synthesized CoIr catalyst is a promising low-noble-metal electrocatalyst for alkaline water electrolysis by significantly reducing noble-metal usage while maintaining high OER performance.