New figures show of the over 3,000 primary schools across the country, almost one in five is in an overcrowded classroom.
The OECD estimates the average primary class size in developed countries is 21.
However that's not the case here.
A Dublin primary school has the highest average class size.
Scoil Mobhí in Glasnevin had an average of 31.9 pupils in each of its classrooms.
Nationwide, one in five primary students are being taught in classes of 30 or more.
Two schools in Galway and one in Donegal were teaching pupils in class sizes of 42.
Kerry children were most likely to be in overcrowded classrooms.
Other counties with pupils in very large classes were Limerick, Westmeath, Kilkenny and Carlow.
The raw data shows that Wicklow's national school students are being taught in class sizes averaging at 25.