Spin waves are promising information carriers for analog and wave-based computing, where functionality relies on compact and precisely engineered scattering landscapes. Focused ion beam (FIB) irradiation enables such control by locally tailoring the spin-wave dispersion in yttrium iron garnet (YIG). However, a non-monotonic dependence of the spin-wave wavelength on increasing ion dose hinders predictive landscape design. Here, we present an experimentally validated framework that explains this non-monotonic spin-wave steering by linking phenomenological strain-induced anisotropy to its magnetoelastic origin. Irradiation-induced lattice dislocations drive elastic and plastic deformation, which evolve into partial amorphization, each stage contributing distinctly to the dispersion behavior. We combine post-irradiation wet-chemical etching and atomic force microscopy (AFM) to quantify thickness changes, and track the dispersion in etched regions using time-resolved magneto-optical Kerr effect (trMOKE) microscopy. Fitting the data to the Kalinikos–Slavin formalism with an added effective magnetoelastic field isolates contributions from elastic and plastic deformation. Validation is achieved by mapping the deformation evolution onto a three-phase scenario based on SRIM simulations, reproducing the extracted field trends, and by consistent strain tensor and micromagnetic analyses. These results establish a physical basis for FIB-engineered graded-index (GRIN) spin-wave landscapes and magnetoelastically programmable magnonic devices.