Mean length of utterance and the acquisition of Irish. 
One of the most widely used indices of language development is a measure of utterance length in morphemes (MLUm).  This study examines the applicability of MLUm to the acquisition of Irish.  MLUm was calculated for data from Cian, aged 1; 11-3; 0.  Even when an attempt was made to 'assume the maximum' by counting all possible morphemes, the correlation between a morpheme MLU (MLUm) and a word count MLU (MLUw) was very high (0.99).  This points to MLUw being as effective a measure of Irish development as MLUm, as well as being easier to apply and more reliable.  MLUw was calculated for the two younger children in the study (Eibhlís 1; 4-2; 1 and Eoin 1; 10-2; 6).  An examination of the relationship between the three children's MLUw values and their grammatical complexity as measured on ILARSP (the Irish adaptation of LARSP) indicates that MLUw is a useful preliminary index for early development in Irish.  However, further data are necessary to check whether MLUw loses its predictive relationship with grammatical complexity after a certain point.  The study emphasizes the caution necessary in applying MLU to languages whose acquisition has not hitherto been studied, and underlines the role of MLU as a preliminary measure, which must not be overinterpreted.