A new effective medium theory is introduced to describe the optical properties of a two-dimensional array of metallic nanoislands. This model which takes into account both the nanoisland orientation and their shape distribution is successfully used to interpret the ellipsometric measurements performed on gold nanoislands sputtered on a silicon substrate. By coupling ellipsometry to atomic force microscopy measurements, we show that the growth mechanism involves a Volmer-Weber growth mode. The optical anisotropy of uniaxial films was attributed to in-plane preferential self-orientation of gold nanoislands. Finally, we demonstrate that the optical birefringence and dichroism of nanoisland layers can be tuned during the film growth and are due to the splitting of the plasmon resonance into two modes: the transversal and the longitudinal modes of gold nanoislands.