Almost one in four primary school children were in classes of 30 or more pupils during the last school year, according to new figures published by the Department of Education.
The data shows that the average class size was just under 25 pupils, slightly higher than the previous year.
The figures also indicate that while the number of second-level students rose, there were fewer teachers working in the sector.
Numbers attending primary school rose last year by 10,000, while numbers attending second-level grew by close to 4,000.
However, the number of secondary school teachers employed fell during the same period by more than 400.
This is thought to be as a result of cuts to the allocation of school guidance counsellors.
121,000 of primary school children (23%) were in classes of 30 pupils or more. 9,000 of them were in a class with 35 or more children.
Many of the schools with the largest classes are concentrated in newer, urban suburbs.
The largest school in the country had 977 pupils last year, while the smallest, on the island of Inishturk, had just three.