Almost a third of Irish school pupils are in now 'supersize' classes of over 30 children.
The number of pupils in classes of 30 or more has risen to over 121,000 in the last year - up by almost 25,000 since 2011.
The latest figures from the Department of Education show that schools in Dublin, Cork and Waterford are worst affected
But Peter Mullen, from primary teachers' union the INTO, said smaller communities are also running into difficulty.
"In other areas where there appear to be smaller classes, we must remember that in those classe there are one, two and three class groups.
"So the teacher might be teaching four-year-olds, five-year-olds, six-year-olds and seven-year-olds.
"That's the reason why they need even smaller classes."