Behaviors of a model interfacial system featuring the clustering of discrete, mobile wall charges and a counterion solution arc investigated The results demonstrate that even very small localized charge clusters produce significant effects for the osmotic pressure, effects that are not adequately represented in common colloidal models. We observe a pronounced decrease in osmotic pressure where a certain level of clustering is attained, with potentially significant implications for theories of colloidal stability and biochemical processes at microscopic length scales. The stable level of wall charge clustering, and thus the osmotic pressure, is found to be variable in the addition of an attractive potential, as measured via the excess free energy of clustering.