Nearly one in five students in primary schools last year were taught in classes of 30 or more, with six schools recorded as having classrooms with 40 or more pupils.
The school with the highest average class size in the country was Scoil Mobhi in Glasnevin, where last year it had an average of 31.9 pupils in each of its eight classrooms.
An analysis of data published by the Department of Education on more than 22,700 mainstream classes in more than 3,100 primary schools across the country shows that 19.8pc of all pupils are in classes that could be considered overcrowded.
It reveals that 45pc of all primary schools had at least one classroom of 30 pupils or more in the 2018/19 school year.
Despite large numbers of new teachers being recruited in recent years, many schools continue to experience difficulty in keeping class sizes at recommended levels.
The biggest class size in prim-ary education last year was 42, recorded in three schools - Scoil Naisiunta Rois, Taylor's Hill, Galway; Scoil Naomh Colmchille, Carndonagh, Co Donegal, and Bunscoil Phadraig Naofa, Tuam, Co Galway.
Overall, more than 109,600 out of 553,319 prim-ary pupils in the recent school year were in overcrowded classes.
Children in Kerry were most likely to be in overcrowded classrooms, with nearly a quarter of all pupils in the county in classes of 30 or more.
Other counties with above- average levels of pupils in very large classes were Limerick, Westmeath, Kilkenny and Carlow.
In contrast, only 16pc of pupils in Cavan and Roscommon were in overcrowded classes.
The figures show the number of students in classes of 25 or more across the country is more than 347,500 - approximately 63pc of the total.
The data reveals that pupils in schools across most counties in Leinster as well as Waterford are more likely to be in classes of 25 or more than pupils from other parts of the country.
However, the Department of Education figures also highlight how average class sizes nationally have been falling continuously since 2015/16, when they stood at 25.4 pupils per class.
They decreased further in the recent school year to 24.3 - down from 24.5 in 2017/18 - but are still relatively high by international standards.
It reached its lowest point in 2008 at 23.8.
The average class size decreased by at least one pupil per teacher in 925 schools last year, but increased by at least one pupil per teacher in 753 schools.
According to the OECD, the average class size in most dev-eloped countries is 21.
There is no statutory limit on the size of general classes, though a department circular in 1990 stated that "appropriate learning experience is difficult to achieve when classes consisting entirely of mainly four-year-old children exceeds 25".
Education Minister Joe Mc- Hugh said Budget 2019 would allow for the numbers employed in schools to reach their highest ever level.
He said it was the third year of major investment in educ-ation, with the department's budget increasing by 6.7pc or €674m on last year.
"Over 1,300 additional posts will be funded, including more than 370 teaching posts, to cater for growth in student population and additional special classes," said Mr McHugh.
The Department of Education said the staffing schedule for the current school year at primary level was operating on a general average of 26 pupils for every teacher.
It is estimated that lowering the primary staffing level by one student per teacher would cost around €13.5m a year.
The Irish National Teachers' Organisation (Into) said class sizes remained "far too high" and called for levels to reach the EU average class size of 20.
"With only 11pc of children learning in a class of fewer than 20 students - the EU average - it's clear they are being short-changed," said an Into spokesperson.
"Smaller classes support inclusion and allow for more individual attention.
Into said falling numbers due to attend primary schools over coming years presented "an ideal opportunity" to further lower class sizes.