This paper readdresses the bounds between rhythm and constituency. It argues in favor of an arboreal representation of the metrical grid in which both metrical prominence, that is, grid marks, and prosodic categories are conflated into the same dimension even at the level of the syllable. These constituentized metrical grids are subject to branchingness constraints on heads and dependents. The research focus is on Munster Irish stress, which illustrates an intricate system of stress assignment. Stress in Munster Irish is assigned to the first syllable in strings containing light (L) syllables, ˈLLLL. Sequences of a H syllable followed by a L syllable always attract primary stress to the H syllable regardless of the position of the sequence within the phonological string, ˌLLLˈHL (cf. ˈLLLˌHH). These facts suggest that uneven trochees (ˈHL) always attract primary stress and therefore might exist as a legitimate metrical grouping. Initial primary stress is also avoided if the third syllable counting from the left edge of the word is H. Thus, a word like /LLH/ is parsed with optional initial secondary stress and primary stress on the H syllable, ˌLLˈH (cf. ˈLLLˌH). The contrast between ˌLLˈH and ˈLLLˌH suggests that some kind of trimoraic constituent determining the location of word stress is necessary. (see all these references for older sources). 2 The data we present come from Iosad (2013), unless otherwise specified. In MI long vowels and diphthongs, but not syllables closed by a consonant, count as heavy (H). 3 Morphological structure does not affect the position of stress. Stress is initial in words that contain no H syllable in the first three syllables of the word (1a–c), and no initial light (L) syllable can be stressed if immediately followed by a H syllable (1e–g), meaning that H syllables attract stress. Sequences of a H and a L syllable attract stress to the H syllable (2). In a sequence of two H syllables word-initially, the second H syllable receives main stress (3). If a H syllable is preceded by a word-initial sequence of two L syllables, primary stress falls on the H syllable (4). 1 Modern Celtic languages are divided into two subfamilies: Gaelic and Brittonic. The Gaelic languages comprise Irish, Scottish Gaelic and Manx. The Brittonic languages include Welsh and Breton. 2 We abstract away from palatalized consonants in our phonetic transcriptions. 3 The sequence [ax] counts as a heavy syllable only if its nucleus forms a second syllable. For other cases of exceptional stress, see Iosad (2013).