The Department of Education spent €282 million on the purchase of modular accommodation last year, an Oireachtas committee has heard.
Separately, over the past three years, the department spent €113m renting prefabs, the Committee of Public Accounts (PAC) was told.
Secretary-General of the Department of Education Bernie McNally said that "temporary accommodation is sometimes needed" to meet the needs of students, hence the renting of prefabs.
However, she said that the use of modular buildings for "permanent accommodation" is "encouraged internationally".
The use of modular units is a "strong, robust, permanent solution" which has received "very positive feedback", the head of the department's Planning and Building Unit Hubert Loftus said.
Ms McNally insisted: "We always look at the most sustainable measure possible the most cost-effective measure possible."
She revealed that the cost of renting prefabs in 2021 was €29m, rising substantially to €41m in 2022 and up again to €43m last year, bringing the total rental cost for those years to €113m.
During that period, there was "a massive increase in demographics", she told Sinn Féin TD John Brady and the department had "to respond to Ukraine, to special educational needs, and the spike that we've seen in demographics".
"There's a real failure here to plan properly," Mr Brady said, prompting Ms McNally to "respectfully disagree".
She pointed to "nearly a million children and young people in the system at any one time".
There are 120,000 to 130,000 entering primary education from infant classes each year, "the vast, vast, vast majority" of which "do get a school place, because we are planning years out".
The department spends almost a fifth of its accommodation budget on the purchase of modular units, Ms McNally revealed.
Mr Brady said that the modular units were being put "on land not owned by the department", which has to be rented and so "is temporary in nature".
"In most instances, it's owned by patrons, we're not paying a rent on it," Ms McNally responded.
She said that a "huge amount of work" had been done to reduce the number of prefabs which the department is renting, particularly those that are being used over the long-term.
In February 2021, there were 396 prefabs which had been rented for more than five years.
By last month, those long-term rentals had fallen to 219.
"The quality of the education system in Ireland is the envy of around the world," Ms McNally said, adding that the high quality of Irish education is backed up by "hard data".
