Catholic and Church of Ireland schools have better teacher-student ratios
The type of school that a pupil attends can have a big influence on the number of children in class.
Location can also play a major factor in class sizes, with Leinster pupils more likely to face overcrowding.
Interdenominational (26.8) and multi-denominational schools (25.5) have higher average class sizes than both Catholic schools (24.3) and Church of Ireland schools (23.1).
Schools in disadvantaged areas - DEIS schools - have an average class size of 21.3 pupils compared with 25.2 in non-DEIS schools.
The largest primary school in the country is St Mary's Parish School in Drogheda, Co Louth, which had 1,103 pupils enrolled last year in 40 classes.
Three schools had just three pupils registered last year - St Columba's on the Co Mayo island of Inishturk; Bofield National School in Bofield, Co Mayo; and the Educate Together National School in Fermoy, Co Cork.
Data on fee-paying primary schools is not available.
Overall, more than 109,600 out of 553,319 registered primary pupils in the last academic year were in overcrowded classes.
Young schoolchildren in Kerry were most likely to be in overcrowded classrooms, with almost 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.