The effect of swift heavy ion irradiation of Fe-Tb multilayers has been studied using x-ray reflectivity and conversion electron Mössbauer spectroscopy measurements. The energy deposited in the multilayer in the form of electronic excitations results in significant modification of the interfaces. During 80 MeV Si ion irradiation the interfacial roughness is found to increase linearly with irradiation dose. Mössbauer measurements provide evidence of segregation of Fe and Tb atoms in the intermixed region, which may be the cause of the observed increase in the interfacial roughness. The as-deposited multilayer exhibits a large perpendicular magnetic anisotropy, which shows a monotonic decrease with irradiation dose. The observed decrease in perpendicular magnetic anisotropy is a combined effect of the stress relaxation in the bulk of layers and an increase in the interfacial roughness. 150 MeV Ag ion irradiation causes additional intermixing at the interface, accompanied by a large reduction in perpendicular magnetic anisotropy.