In agreement with other responses submitted, yes, you can have reduction mammaplasty after chest and/or breast irradiation. The amount and location of the radiation may affect surgical decisions and technique. Low dose radiation probably immaterial. Higher dose concentrated radiation may result in localized fibrosis and potential impaired healing, with a higher chance of wound dehiscence, delayed healing, poor scarring, infection, and other problems. If only one breast was irradiated, these risks do not apply to the non-irradiated side. Your surgeon may adjust his or her technique to avoid vascular compromise.
Not all radiation is equal. Ask your plastic surgeon about your specific situation.