Detailed first-principles calculations are performed to predict the favoured adsorption sites, ground state magnetic alignment, and electronic properties of Mn on a Ge (111) clean unreconstructed surface, in order to shed light on the physical mechanisms which drive adsorption. In this geometry, Mn is predicted to adsorb on the surface interstitial site with a ferromagnetic half-metallic ground state. Nevertheless, an energetically competing site is the substitutional subsurface site, which leads to a nonmagnetic ground state due to its large site coordination. Possible competing adsorption paths are also discussed, which, thanks to surface effects, may lead to incorporation into different sites.