The ASTI are warning that problems with the recruitment and retention of teachers could have a serious knock-on impact on the teaching of some core subjects.
500 teachers are meeting again in Cork for the second day of the annual ASTI conference.
Concerns about what the union describes as low pay were top of the agenda yesterday, with some delegates speaking about how the rising cost of living is affecting them.
The conference heard how some city schools are finding it hard to recruit staff due to the price of rent, while some teachers are leaving the profession to work elsewhere.
Anne Loughnane is a teacher and ASTI representative for West Cork and the city.
She outlined to RedFM News how the recruitment crisis is leading to problems for some schools.
"There are many vacancies in the country where there has been no applicant for the job, and that is particularly worrying.
"In particular three subjects are very heavily affected - Irish, maths and home economics. It's almost impossible to get substitutes or teachers into those subjects.
"IN the case of Irish and maths, those are mandatory subjects and if there are no teachers to teach them it's becoming a crisis.
"It is a crisis at the moment, and it will get worse going forward unless the issues that are causing teaching to be less attractive are addressed by the department and the minister."
