In this paper I argue that a subpattern of Irish plural allomorphy should be analyzed as output optimizing in character. Specifically, I claim that stress-sensitive alternations between the plural suffixes -(e)anna and -(e)acha are conditioned by constraints on metrical well-formedness. This analysis connects with independent facts about the the prosodic prominence of [ax] sequences in Irish phonology. I further argue that an explanatory analysis of these patterns must make use of the notion of surface optimization. Alternative frameworks that eschew surfaceoriented optimization mechanisms fail to account for synchronic and diachronic properties of the Irish plural system.