The ab initio atomistic thermodynamics technique was used to calculate the surface free energy diagrams. These diagrams were obtained for hydrogen over the unreconstructed low-index iridium surfaces. Results indicated that the Ir(110) surface acquired the highest hydrogen intake among the low-index surfaces with a coverage of up to 2.33 ML. However, high-pressure hydrogen conditions were essential to maintain the stability of this phase. The Ir(100) surface required −0.81 eV of hydrogen chemical potential to produce a stable H/Ir phase with a 1.0 ML coverage. This potential represented the lowest value among the low-index surfaces. Phase diagram calculations of H/Ir(111) illustrated that the 0.75 ML phase has the highest stability at low to moderate hydrogen pressures. On the other hand, the 1.25 ML-H/Ir(111) phase became the most stable under high-pressure conditions. Finally, Wulff construction was implemented to predict the shapes of iridium nanoparticles in a hydrogen environment. Predictions indicated a truncated-octahedron shape for the nanoparticles irrespective of the value of the hydrogen chemical potential.