The hysteresis curve at very low temperature of Co nanoparticles organized in a two-dimensional network on a graphite substrate is determined both experimentally and from a numerical calculation. The Co particles are modeled as spherical particles of uniaxial symmetry and the easy axes are randomly distributed. We focus on the effect of the dipolar interactions between particles on the hysteresis loop: the magnetization curve is thus compared to the case of non-interacting particles, which corresponds to particles dispersed in solution at vanishing concentration. The magnetization curve is calculated for two orientations of the applied field: normal and parallel to the substrate surface. The ratio between the remanence magnetizations obtained with the field, normal and parallel to the surface, appears to be a convenient parameter to estimate the importance of the dipolar interactions. A satisfactory agreement with experiment is obtained for this ratio.