The global minimum of IrB₁₂^- is a C_3v symmetric bowl-like structure in which the Ir atom is located on the concave side of the bowl, similar to its lighter congeners, CoB₁₂^- and RhB₁₂^- clusters. Although all these MB₁₂^- (M = Co, Rh, Ir) clusters show dynamical behaviour, analogous to that of the so-called 'Wankel motors', the energy barrier for the rotation of the inner B₃ ring within the peripheral B₉ ring is the lowest in the IrB₁₂^- case (5.0 kcal mol^-1 only). The geometrical feature along with the lower interaction energy between B₃ and MB₉ moieties are responsible for a smaller rotational energy barrier in IrB₁₂^- than those in CoB₁₂^- and RhB₁₂^- clusters.