Primary class sizes could grow even larger to pay for the U-turn on special needs education cuts that were announced last week.
Class sizes are already the second largest in the EU, with primary schools allocated one teacher for every 28 pupils.
But it is feared this could increase to one teacher for every 30.5 pupils to cover the cost of the reversal.
The threat of more overcrowded classrooms comes as Education Minister Ruairi Quinn seeks to recoup the €21m cost of maintaining resource teacher support for children with complex disabilities.
That is on top of €44m in savings that Mr Quinn is already committed to bringing to the table for the October Budget.
But Mr Quinn will have a battle on his hands with the Irish National Teachers' Organisation (INTO), which warned last night against any attempt to increase class sizes.
The Government's U-turn on special needs involves bringing forward to September the appointment of almost 500 resource teachers to deal with a known additional 12pc demand.
It had been intended to keep overall resource teacher numbers capped at 5,265 and to reserve 492 posts to deal with new cases.
However, keeping a cap on resource teacher numbers, with 4,100 more pupils requiring assistance, would have left individual children with a reduced service and, in the face of an outcry, Mr Quinn backed off.
Now, in order to pay for it, the question of raising the pupil-teacher ratio (PTR) at primary level has moved centre stage in budget discussions in the Department of Education, it is understood.
Requiring primary schools to have another 2.5 pupils for each teacher allocated translates into a cut of 500 mainstream posts, the equivalent of what was conceded last week.
While the reversal of the cuts affects both primary- and second-level schools, it is understood that the focus is on the primary PTR.
There are three ways of measuring teaching resources in schools, starting with the department's staffing schedule, which currently allocates one teacher for every 28 pupils.
The more familiar pupil-teacher ratio is found by dividing the number of pupils by all the teachers in the system, including non-class teaching staff such as administrative principals, resource teachers and home school teachers.
At primary level in Ireland, this is 16 pupils per teacher, compared with the EU average of 14.
Average class size is measured by dividing the number of pupils by the number of class teachers. In Ireland, this is 24 compared to an EU average of 20. While 24 is the average, more than 100,000 primary pupils are in classes of more than 30.
National Parents Council Primary chief executive Aine Lynch said they would be watching the Budget deliberations very closely, and decisions would need to make the least possible impact on children in the classroom.
INTO strongly condemned the threat to increase class sizes and warned it would oppose any attempt to put more pupils into "already over-crowded primary classrooms".
The union's general secretary, Sheila Nunan, said research showed clearly that small classes in the early years of school benefited children.