Nearly 20 per cent of primary school pupils are being taught in classes with 30 or more children.
New figures show of the over 3,000 primary schools across the country, almost one in five are in overcrowded classrooms.
The OECD estimates the average class size in most developed countries is 21, while the Department of Education recommends class sizes of no more than 25.
Two schools in Galway and one in Donegal were teaching pupils in record class sizes of 42.
INTO General Secretary John Boyle says government investment is urgently needed:
Concerns over class overcrowding at primary schools