The amount of time devoted to the Irish language in 3,000 primary schools is to be scaled back by 30 per cent from next Monday.
Teachers say this will represent the first cutback in Irish teaching at primary level since the foundation of the State.
The time spent teaching Irish is being reduced to accommodate new subjects such as science and social studies.
At present, primary school teachers are obliged to spend about 20 per cent of all classroom time teaching Irish, or about five hours per week.
From Monday, teachers must teach Irish for a total of three and a half hours per week.
By contrast, in the 1950s most teachers had to teach all infant classes in Irish. One 1986 survey of 6th class teachers found they were spending an average of five hours 40 minutes per week teaching Irish.
Last night, the INTO general secretary, Mr John Carr, said that the commitment of primary teachers to the Irish language must be recognised.
"Other sectors in the State have not supported the Irish language to the extent that they could and so for a large percentage of the population Irish is only associated with schools. It is unrealistic and unfair that schools alone should be expected to develop the language."
The new subjects on the curriculum include social, environmental and scientific education and social, personal and health education, which incorporates anti-bullying, civics and other programmes. Schools will also enjoy two hours per week "discretionary curriculum time".
From next Monday, teachers will be teaching a new much-praised Irish curriculum. This places a new onus on oral and practical language skills, replacing the old emphasis on grammar and written work.
One source said: "There is a realisation out there that the old curriculum has been failing. We need to spark a new interest in the language."
Under the new curriculum, pupils will learn to converse on topics such as shopping, TV, food and clothes.
The popularity of higher-level Irish has declined dramatically in second-level schools in recent years. This year, only 15,000 students took higher level at Leaving Cert level.
More than 35,000 took either ordinary level or foundation level Irish.
The new Gaeilge curriculum says the emphasis will be on the child learning to "use the language as an effective means of communication. Topics are based on the children's own interests, concerns and needs and children are encouraged to speak the language in real contexts and situations".
The Department of Education promises that the emphasis in the new course will be on "enjoyment and on using the language in activities such as games, role-playing, sketches and drama".