We successfully prepared fine particles of the diluted magnetic semiconductor (DMS), Cd1-xMnxS (x = 0.20), in Langmuir-Blodgett (LB) films, and then examined the ion irradiation effect on their surface electronic states. From in-situ observation of the emission induced by 2.0 MeV H+ bombardment, the ion irradiation was found to decrease the intensity of the lower-energy emission via deeper defect states under the condition that the intrinsic Mn2+ state remained. This clearly shows that ion irradiation will provide good surface passivation to Cd1-xMnxS fine particles as determined in our previous study of CdS fine particles. These surface-treatment effects of ion irradiation appeared to be prominent in the nanoparticles whose surface-to-volume ratio is much larger than that of the bulk crystals.