New figures published by the Department of Education show just under 24% of all pupils in primary schools where teachers have class sizes of 30 or more students.
The average class size in most developed countries is around 21, according to the OECD.
Education Minister Richard Bruton said the staffing schedule in the current school year at primary level is for a pupil-teacher ratio of 1:28.
However, the actual average is usually lower and was just under 25 in the 2014/15 school year as lower thresholds apply for the country’s 646 Deis schools ranging from one teacher for 20-24 pupils depending on the type of school.
The figure for the current school year has yet to be released.
Nevertheless, two-thirds of Irish primary schoolchildren are in class sizes greater than 25.
A total of 540,955 pupils are enrolled in primary school in the current school year — up from 532,993 in the 2014/15 term — an increase of 1.5%.
A breakdown by county shows the most crowded classrooms are found in Meath, Carlow, Louth and Limerick.
Almost 31% of pupils in Meath are in classes with average sizes in excess of 30 and almost 80% are in classes of 25 or more.
The least crowded primary schools are located in Leitrim with just under 15% of pupils in classrooms of 30 or more, followed by Cavan, Roscommon, Mayo and Kerry. Fewer than 55% of schoolchildren in Mayo are in class sizes of 25 or more.
In Dublin, which accounts for a quarter of almost all primary schoolchildren, a total of 22.6% are in class sizes of 30 or more. In Cork, the figure is 25.8%.
The new Programme for Partnership Government, makes a commitment to reduce the pupil-teacher ratio in primary schools but does not specify a figure.
Provisions made by the previous Fine Gael-Labour coalition in Budget 2016 will see the staffing schedule for the 2016/17 school year operate on the basis of one classroom teacher for an average of every 27 pupils — a reduction of one on the current year.
In response to a recent parliamentary question by Sinn Féin TD Carol Nolan, Mr Bruton said it was estimated that 250-300 additional teaching posts would be required to lower the average class size by one pupil. The cost of providing additional classroom accommodation for each one point adjustment to the staffing schedule is estimated to be between €34m and €40m.