The phase diagram of hardcore bosons on a triangular lattice with nearest neighbor repulsion is determined using quantum Monte Carlo simulations. Special attention is devoted to the stability of supersolid phases. Similar to the same model on a square lattice we find that for densities $rho2/3$ a supersolid phase is unstable and the transition between a commensurate solid and the superfluid is of first order. At intermediate fillings $1/3Stuttgart