The transfer of a tetramethylammonium ion across the water/nitrobenzene liquid/liquid interface is studied at 300 K using molecular dynamics computer simulations. The potential of mean force, calculated using two different techniques, shows that the transfer is accompanied by a relatively small change in the free energy, in agreement with experiments. A comparison of the potential of mean force with a continuum electrostatic model shows reasonable agreement. The mechanism of ion transfer across the interface under conditions of an external electric field is investigated by nonequilibrium molecular dynamics calculations. Despite the fact that this process has only a small free energy driving force, several of the structural and dynamical characteristics of the system during the transfer are similar to those observed in the study of the transfer of small ions with a large free energy of transfer.