An algorithm is presented for representing irregular particles as clusters of overlapping spheres, for use within discrete element method (DEM) simulations of particulates. The algorithm is sufficiently fast to be implemented on desktop computers. Although volume and moment of inertia comparisons of resulting clusters with prototypical geometric data show that in the region of 50 to 100 spheres may be needed to represent a particle, actual DEM simulations of an unstable pile of simulated particles show that only 10 or so spheres may be sufficient to capture the mechanics of the system.