Schools are under "enormous strain" with hundreds of teaching posts still unfilled ahead of the new term, the Association of Secondary Teachers in Ireland has warned.
In a statement, ASTI said there are 416 post-primary teaching vacancies advertised at present on recruitment website educationposts.ie
Among the principals struggling to recruit teachers before schools reopen at the end of this month is Louise Ronan, Principal of Coláiste Bríde in Clondalkin, Dublin.
Speaking to RTÉ's Morning Ireland, she explained: "We have a number of vacancies right now. Home economics, for example, is one of the most difficult posts to fill.
"I’ve advertised five or six times already. We had such difficulties last year that I taught some exam classes myself.
"Our school has almost 1,000 pupils but instead of prioritising being a leader for Coláiste Bríde I ended up back inside a classroom – there was no other option.
"These were Home Economics students in exam years and they absolutely required a teacher."
Ms Ronan added: "Other areas where I am really struggling to find teachers include guidance counselling and Special Educational Needs.
"I have had zero applicants for some positions."
ASTI said a survey published by the union and Red C earlier this year found that there were unfilled teaching vacancies in almost half of all second-level schools.
ASTI General Secretary Kieran Christie called on Minister for Education Norma Foley to urgently address what he called the teacher supply "crisis".
In a statement, the Department of Education said it acknowledged that problems persist when it comes to sourcing teachers.
It said extra supports were now in place for post-primary schools, among them a pilot teacher-sharing scheme allowing two schools to collaborate on high-priority subjects.
Other measures include an upskilling programme to boost the number of Irish language teachers and encourage retired teachers to take up short-term substitute positions.
Speaking on RTÉ's Morning Ireland, Mr Christie said there are still some issues around a Government policy to pay newly qualified teachers less than the teachers that came before them, which, he said, is now "washing its way through the system".
He added: "The fact of the matter is that teaching is no longer the attractive profession it used to be. There is very limited promotional prospects within teaching."
Mr Christie said a large number of qualified teachers are also taking career breaks and heading to countries such as Dubai, Canada and Australia.
'Challenges in Dublin are felt in other cities'
Minister for Public Expenditure Paschal Donohoe says he accepts the general challenges in recruiting teachers at the moment.
When asked at a news conference today if he would favour a Dublin allowance for teachers, he said the challenges in Dublin are felt in other cities.
He said the Government is looking at a range of issues and has previously sanctioned pay increases for teachers through the public sector pay deal.
He said they will also facilitate the return of retired teachers.
Minister for Higher Education Simon Harris said the issue is not unique to teaching.
Additional reporting: Sinead Spain
