Previous research has demonstrated the validity of parent report as an assessment of early vocabulary and grammatical development across a range of languages.This study adapted a well-known tool, the MacArthur-Bates Communicative Development Inventories (MCDI), to Irish (ICDI), in order to measure vocabulary and grammatical development longitudinally across a sample of 21 children. Results from the parent checklists were validated against spontaneous language samples. Correlations demonstrated high concurrent validity for ICDI reported vocabulary with measures of lexical diversity derived from the spontaneous samples. In addition, ICDI measures of grammar also correlated highly with various indices of grammar from the spontaneous samples. Although both ICDI and spontaneous vocabulary measures were highly correlated, the ICDI checklist includes a broader range of language skills and so would seem to capture the range of language ability more comprehensively than direct observation. The findings have implications for developing parent report measures of early language development in highly inflected languages. © The Author(s) 2010.