|Abstract:||Irish is characterized by a process of lenition, by which (among other changes) the coronals t, d, s become h, G (gamma), h under certain morphosyntactically determined circumstances. Lenition of coronals is blocked (i.e. t, d, s remain unchanged) after other coronal consonants in certain domains, a phenomenon known as coronal fusion (CF). In a subset of CF domains s changes to t rather than remaining s, a phenomenon known as s-fortition. In this paper, it will be shown that the domain of CF and s-fortition is the (recursive) prosodic word, as these two processes are found in right-headed as well as left-headed compounds, but not in other (noncompound) left-headed complex NPs. An optimality-theoretic analysis reveals that CF and s-fortition are motivated by the same constraint ranking: the phonological requirement that coronal consonants be followed by other coronal consonants is more important than the selection of the morphologically correct mutation grade of a word.