Ultra-small CoPt nanoparticles (NPs) with a mean diameter of 1.3 nm (around 100 atoms) were deposited on a thin 5 nm self-supported amorphous carbon membrane. The effects of focused irradiation with a newly developed Ga(+) ion source were studied by transmission electron microscopy. While the overall coverage of the NPs remained constant, the mean diameter and the density of the NPs evolve in the dose range from 5 x 10(13) to 1 x 10(15) ions cm(-2). The local tuning of the size and density of CoPt NPs by means of ion irradiation could be used in magnetic data storage applications.