The report, compiled by inspectors from the Department of Education, found the standard of teaching and learning was 'fair to poor' in half of all classes observed.
It observed significant difficulties in 8% of classes.
The inspectors described the findings as disquieting.
The report also says it is worrying that almost a quarter of teachers had weaknesses in their own language competency and 9% had significant deficiencies.
The study also found that formal teaching of listening skills was weak or fair in more than half of classrooms evaluated.
It criticised a lack of variety and purpose in listening activities and says the reading material used presents very little challenge.
The report also highlights the fact that in almost one-third of classrooms, pupils are still being taught Irish through English.
Inspectors based their findings on an evaluation of teaching and learning in 159 classrooms across 40 mainstream schools.
Their report, Irish in the Primary School, is one of two reports into the teaching of the Irish language published today by the Department of Education.
