This paper argues that initial consonant mutations in Irish should be separated into two groups based on their domain of application. There are morphological mutations which are confined to a morphosyntactic domain—namely the syntactic phase or spell out domain (Lenition). There are also phonological mutations which are more accurately described as prosodic changes used to avoid marked phonological sequences like vowel hiatus and are confined to a phonological domain—a morpheme boundary within a prosodic word (h-Prothesis). More broadly this paper provides evidence that nonconcatenative types of morphology are constrained to definable domains and should be treated differently than phonological alternations that are much more local (between segments/syllables). In addition, I put forth an argument for a phonological grammar that operates cyclically over the spell out of syntactic phases. Finally, this paper fills a gap in the Irish literature with respect to locality domains for initial mutations.