One-quarter of all children starting school this week are in classes of 30 or more pupils, despite Government pledges to provide classrooms with a maximum of 20 children, according to the Irish National Teachers' Organisation.
Fewer than one in five pupils will be accommodated in classes where the 20:1 pupil-teacher ratio promised by the Government four years ago is implemented, according to INTO general secretary John Carr.
Schools across Dublin's suburbs and so-called "satellite towns" are suffering the most serious overcrowding, with areas such as Lucan, Blanchardstown, Castleknock and Clonee particularly affected.
In Scoil Mhuire in Lucan, where a new prefab is due to arrive today for the start of the school term, 58 junior infant pupils will be accommodated in two classrooms.
With 30 children in each of the first classes and 31 in fourth class, school principal Henry Thynne said that the school was "bursting at the seams".
The only Church of Ireland school in Lucan, St Andrew's National School, received 112 applications for 30 junior infant places. More than 230 children are accommodated on the 0.8 acre site, which has class numbers in excess of 30 children.
Principal Inez Cooper said that the general purpose room in the school had been used as a classroom since 1985 in an attempt to accommodate as many children as possible. However, this had made the teaching of physical education, music and drama increasingly difficult.
In addition, the school's language and learning support teachers had been forced to teach in a store room, and in another room once used as a library, due to the lack of available teaching space.
"We hope to get two new prefabs, but there has been a delay with the planning authorities. If we got the prefabs, it would free up the assembly hall [ general purpose room] and the store room," Ms Cooper said.
In the Griffeen Valley Education Together school, where two junior infant classes of 30 children start today, principal Thomas O'Dulaing said that the summer months had been "terribly heart-rending" for local families.
"The State has been allowing tens of thousands of houses to be built here with no facilities being put in place. Families have been left in an appalling state," he said.
Mr O'Dulaing described as "shameful" the fact that junior infants would be taught in classes alongside 29 others when the Government had made a promise of providing classes of 20 children.
Michael Maher, principal of St Thomas's junior national school, where most classes consist of between 28 and 30 children, echoed similar concerns.
"If you were to take a ruler and draw a line from Lucan to Clondalkin, you would see that it's all housing estates. There has been no mixture in urban planning. With so many new estates, you have families moving into the area at the same time and looking for a school at the same time," he said.
Lucan, Co Dublin: A case study of overcrowding
ST ANDREW'S CHURCH OF IRELAND NATIONAL SCHOOL
Junior infants: One class of 30 children. Waiting list consists of 112 enrolment applications
First, second and third class: Over 30 children in each, with one class accommodating 32 children. The general purpose room has been used as a classroom since 1985, which hinders its use for physical education, music and drama classes. Learning support teachers are based in a store room and another small room.
GRIFFEEN VALLEY EDUCATE TOGETHER
Junior infants: Two classes of 30 children. Over 500 applications received for 60 places in junior infants.
Other classes: All classes have between 28 and 30 children.
SCOIL MHUIRE
Junior infants: Two classes of 29 children. Senior infants: Two classes of 29 children. First class: Two classes of 30 children. Fourth class: 31 children
ST THOMAS'S JUNIOR NATIONAL SCHOOL
Junior infants: Four classes of 28 children. All classes: Between 28 and 30 children.
ARCHBISHOP RYAN JUNIOR SCHOOL
The school's disadvantaged status should allow for 20 children to one teacher. Junior infants: Six classes of 25; 55 families on waiting list
ST MARY'S BOYS' NATIONAL SCHOOL
Sixth class: Two classes of 31 and 32 children. Junior infants: Three classes of 27. Senior infants, third class and fourth classes: 30 children in each.