One in five students in Irish primary schools was in a class of more than 30 pupils last year.
The Dept of Education warns that the figure is expected to rise this year as a result of budget cuts in teacher allocation.
It has compiled head counts from every primary school in Ireland.
The figures show an average class size of 24.3 children, which is three more than the European average.
In all, 92,000 children are being educated in classes of 30 or more pupils and 5,000 are being educated in classes of more than 35.
The Irish National Teachers' Organisation said the figures show clearly the problem of overcrowded classes.
