Temperature programmed desorption is used to investigate ammonia oxidation by pre-adsorbed oxygen on planar Ir(210) and faceted Ir(210) with different facet sizes. Evidence is found for formation of N2O from both surfaces, which is the first observation of N2O production in catalytic ammonia oxidation under UHV conditions (< 1 x 10(-9) Torr). The selectivity of the reaction to N-2, N2O and NO can be tuned by oxygen pre-coverage, surface structure, and facet size. The reaction exhibits strong structure sensitivity on faceted Ir(210) versus planar Ir(210) and moderate size effects on faceted Ir(210) for average facet sizes of 5-14 nm.