Composite solid electrolytes between organic ionic plastic crystals (OIPCs) and polymers represent a promising alternative for safer energy storage devices such as batteries. A detailed understanding of the phase behavior, structure and ion dynamics in the materials is very important in their design. Here we studied a novel composite electrolyte formed between the OIPC N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)amide (C2mpyrFSI) and different amounts of polymer nanoparticles that consist of a crosslinked poly(methyl methacrylate) including functional co-monomer lithium 1-(3-(methacryloyloxy)propylsulfonyl)-1-(trifluoromethylsulfonyl) imide (LiMTFSI). Characterisation by thermal analysis, X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy shows increased structural disorder in the OIPC after incorporating the polymer nanoparticles, which led to an increase in ion conductivity for the composite with 10 v% of polymer nanoparticles. A detailed study of the composite with 20 v% polymer nanoparticles showed that the Li ion mobility was restricted despite the formation of a disordered interfacial region in the material. A comparison between the properties of the composites based on C2mpyrFSI and N-methyl-N-ethylpyrrolidinium bis(trifluoromethanesulfonyl) (C2mpyrTFSI) is also presented.