In order to teach a language effectively, teachers need to be both proficient users of the target language, and have an explicit understanding of its linguistic structures. This study reports on the linguistic knowledge of Irish of a group of student primary teachers (SPTs) (N = 75) preparing to teach in primary schools in the Republic of Ireland. A C-test was administered as a measure of students' general language proficiency in Irish, and a written grammar test was designed to assess their explicit knowledge of four target forms. Results from the study highlighted a wide variance in student proficiency in Irish and significant gaps in students' explicit knowledge of the language. There was a clear difference between students' procedural and declarative knowledge of the target language forms, with declarative knowledge posing the greatest challenge. SPTs were found to lack knowledge of common metalinguistic terms and to have particular difficulty with the verbal noun clause, a structure which does not map directly from English to Irish. Implications arise for teacher education for generalist language teachers, and for teachers of minority languages in particular.