N-N adsorbate-adsorbate interactions on a Ru(0001) surface are first estimated using quantum mechanical density functional theory (DFT) calculations, and subsequently incorporated, for the first time, in a detailed microkinetic model for NH3 decomposition on Ru using the unity bond index-quadratic exponential potential (UBI-QEP) method. DFT simulations indicate that the cross N-H interactions are relatively small. Microkinetic model predictions are compared to ultra-high vacuum temperature programmed desorption and atmospheric fixed bed reactor data. The microkinetic model with N-N interactions captures the experimental features quantitatively. It is shown that the N-N interactions significantly alter the rate determining step, the most abundant reaction intermediate, and the maximum N*-coverage, compared to mechanisms that ignore adsorbate adsorbate interactions.