More than 100,000 primary pupils are being taught in classes of 30 students or more, while more than 200 are going to school in classes with at least 40 other children, according to the latest Department of Education and Science data.
Despite Government pledges to cut class sizes, a county by county class-size breakdown shows 106,000 - or one-in-four - pupils at primary level are being taught in classes of 30 to 39.
The figures also show that 257,000 students are being taught in classes of 20 to 29.
They show that Dún Laoghaire-Rathdown, Dublin Fingal, Meath and Kildare are the worst areas in the Republic for large class sizes.
More than one in three primary students in these areas go to school every day in classes of 30 or over. By comparison, children in Cavan, Longford, Mayo, Roscommon, Tipperary and Offaly are more likely to be in classes of one to 29 than their counterparts in these areas.
The figures, contained in a written Dáil reply to the Labour Party spokeswoman on education, Ms Jan O'Sullivan, have been strongly criticised by the party and by the Irish National Teachers' Organisation (INTO).
According to the figures 212 children in schools in Dún Laoghaire-Rathdown, Meath, Cork County, Wexford and Belgard in Dublin are going to school every day in classes of 40 and more.
This means a total of 363,566 students - or 85 per cent - are being taught in classes of 20 or over, while 66,000 are being taught in classes of between one and 19. These are the latest figures available and relate to the 2003-04 academic year.
In the 2002 Programme for Government, Fianna Fáil and the Progressive Democrats said that by 2007, they would reach class size targets of fewer than 20 for all children under nine years of age.
However, the Minister for Education and Science, Ms Hanafin, recently admitted that these targets on class sizes would not be met. This was in part because the State's teaching institutions were not producing enough teachers to meet the target, she indicated.
But Ms O'Sullivan said yesterday the figures provided little evidence of progress on reducing class sizes.
Especially when coupled with this week's Budget, they indicated that the Government did not have the political will to tackle the issue, she said.
"Children in large classes aren't going to get the kind of individual attention they will require," she said.
"This Government needs to take specific action in relation to class sizes."
This should include the possible introduction of "maximum class sizes", with classes split in two once they reached a certain size, she added.
Mr John Carr, general secretary of the INTO, said Irish children are in some of the largest class sizes in Europe.
"The last three budgets have failed to reduce class size. The Government is running out of time to implement this promise," he said.
"Failure to reduce class size on the grounds that we cannot afford the teachers is a false economy.
"Reducing class sizes in the early years is a preventative rather than a remedial approach.
"There is no excuse for inaction on this commitment and many parents will wonder why their children are in classes of 30 or more."