Bottom-up growth of microscopic pillars is observed at room temperature on GaN irradiated with a Ga+ beam in a gaseous XeF2 environment. Ion bombardment produces Ga droplets which evolve into pillars, each comprised of a spherical Ga cap atop a Ga-filled, gallium fluoride tapered tube (sheath). The structures form through an interdependent, self-ordering cycle of liquid cap growth and solid sheath formation. The sheath and core growth mechanisms are not catalytic, but instead consistent with a model of ion-induced Ga and F generation, Ga transport through surface diffusion, and heterogeneous sputtering caused by self-masking of the tapered pillars.