Surface-grafted polyelectrolytes provide a versatile way to create functionalized interfaces and nanochannels with externally controllable properties. Understanding the behavior of ions within the brush-like assemblies is crucial for the further development of these devices. We demonstrate that the ion transport through the brushes is governed by the interplay of electrostatic ion-polymer binding and steric effects, leading to a mobility that depends nonmonotonically on grafting density. However, the ion-polymer binding can be modulated by the dielectric properties of the substrate. As a result, surface polarization suppresses ion mobility near insulating interfaces and enhances it near conducting interfaces, even causing a shift from nonmonotonic to monotonic variation with grafting density.