Understanding the short-range molecular motions of organic ionic plastic crystals is critical for the application of these materials as solid-state electrolytes in electrochemical devices, such as lithium batteries. However, The theory of short-range-motions was originally developed for simple molecular plastic crystals and does not take account of strong interionic interactions that are present in organic ionic plastic crystals. Here we report a fundamental investigation of the dynamic behavior of an archetypal example triethyl(methyl)-phosphonium bis(fluorosulfonyl)amide ([P-1222][FSI]) through calorimetry, impedance spectroscopy, synchrotron X-ray diffraction, and solid-state NMR and Raman spettroscopies. For the first time, we shaw the presence of conformational dynamics in the solid state for the FSI anion. We relate the dynamics to a unique second-order displacive phase transition of [P-1222][FSI]. This detailed analysis suggests a new disorder mechanism involving cooperative motion between the cation and FSI anion in the plastic crystal due to strong interionic interactions.