The well-studied phenomenon of vowel lengthening before tense sonorants in Modern Irish (Ó Siadhail 1989, Ó Siadhail and Wigger 1975, Ó Baoill 1979), exemplified in (1), is now well understood as a compensatory process (see for example Ní Chiosain 1991). Tense or long sonorants (L, L', R', N, N', m) trigger vowel lengthening and/or diphthongization. What is not clear about this phenomenon is th...