The effect of ion irradiation on matter is generally thought to involve "radiation damage", i.e., deleterious disordering or even lattice disruption. The purpose of this talk is to show that since the basic features of ion beam interactions with materials are now understood, we are nearing a situation where ion irradiation could become a 'control parameter' (analogous, say, to very familiar control parameters such as temperature or pressure) in experiments or in materials processing. I will show why this is so, and show why and how this special type of 'control parameter' is well-adapted to the structuring and property improvement of nanoscale materials (appropriate ion irradiation can even improve lattice order). My examples will be taken from the area of magnetic nanostructures, where the technique could allow the production of extremely high density recording media as well as entirely new types of sensors. My main message will be that - as often when working with nanosized materials - this is an area where "basic" and "applied" science are often indistinguishable. The physical size of the objects under study is actually on the scale of most of the elementary interactions between their constituents (e.g., the magnetic interactions between atoms, electron mean free paths, etc.), so that new applications often involve very basic and exciting science.