Labour leader Eamon Gilmore says Department of Education figures show that 40,000 Irish children are being taught in prefabs.
Speaking in the Dáil this morning, Mr Gilmore said the figures showed that 2,235 prefabs were being used in more than 800 primary schools.
He said such accommodation costs €35m to rent and asked Taoiseach Brian Cowen to accelerate the school buildings programme.
Deputy Gilmore suggested that given the downturn in residential construction, there was now the capacity to allow for extra school building.
Mr Cowen pointed out that capital spending on school building was now €640m, and that work was prioritised in those areas that needed new buildings most.
Mr Cowen said 4% of the student population are in temporary accommodation.
He said there will always be occasions where such buildings will be part of the solution, but he said they were not the permanent answer.
The Taoiseach also said there was unprecedented capital investment in the schools building programme in the last 10 years.