Sinn Fein MLA Pat Sheehan has called on the Education Minister to do more to ensure the needs of Irish medium education are met.
It follows reports first published by the BBC that Colaiste Feirste is facing accommodation pressures.
The West Belfast MLA said the sector, which is rapidly growing in Northern Ireland with over 7,000 young people learning through Irish here, is facing increasing demand.
The school in west Belfast was set up to accommodate 600 pupils and is expected to welcome 900 this autumn.
“Over 60% of Irish Medium schools reside in so called temporary prefab or modular accommodation. The rapid growth of Irish Medium education is expected to continue with enrolment figures set to hit 10,000 pupils in the coming years," said Mr Sheehan.
"There is a duty on the Department of Education to encourage and facilitate Irish Medium Education and the minister needs to realise this duty isn’t optional, The Minister has an obligation to engage with and address the issues facing Irish medium education.”
The principal of Colaiste Feirste Mícheal Mac Giolla Ghunna said the school is facing an accommodation "crisis" and may have to use its staffroom and sports facilities as classrooms from September.
Built in 1991, Colaiste Feirste is one of only two Irish medium post-primary schools in Northern Ireland.
Mr Mac Giolla Ghunna said the school had been left in "an unprecedented situation" but added: "It's important to emphasise to parents that we will accommodate pupils for next year.
"There are 161 coming to us, and we will accommodate all of them and we will ensure their educational experience is very good in line with the standards in this school.
"But the school library is now a classroom, the technology classrooms are used for modern languages, there's no sixth form common room as that's used as a classroom.
"Store rooms are used for smaller A-Level classes.
"That's not acceptable because it was foreseen and that's what's really frustrating us. The crisis is continuing: at present we will have approximately 900 pupils in September and we estimate conservatively that we'll be over 1,000 in three years time."
The school has been attempting to find a second site in the north or west of the city to run a "satellite" campus but Mr Mac Giolla Ghunna said that had "hit a wall". He said there would soon be a need for a second Irish-medium post-primary in Belfast.
The Department of Education has been asked for a response.