<p>This paper resolves an apparent paradox arising in an autosegmental account of Irish initial consonant mutation: the unexpected distribution of preverbal tense particle <i>d’</i>. Although <i>d’</i> is usually taken to be the “trigger” of mutation, the conditions for its insertion seem to depend on the post-mutation form of the following word. It thus appears that the mutation trigger enters the derivation only after mutation has taken place. I consider two modularity-compatible solutions that differ only in where they place the burden of explanation for this pattern (i.e., on the morphosyntax or the phonology). Using evidence from both modules, I conclude in favor of the “phonological solution”: preverbal <i>d’</i> is an underlyingly floating segment that is inserted in all phonological environments; and its distribution follows from the regular phonology. I thus demonstrate that (i) the Irish preverbal <i>d’</i> pattern is compatible with strict modularity; and (ii) a methodology that commits to a priori modular assumptions can lead to important insights concerning the division of labor at the morphosyntax-phonology interface.</p>