Recruitment and retention difficulties are having a negative impact on schools, with Irish, Maths and Home Economics among the subjects worst affected, according to the Teachers' Union of Ireland.
In a survey of 150 schools, three quarters said they had received no applications for a position in the previous 12 months, while 54% said their school had unfilled vacancies.
Nine out of ten said they believed recruitment and retention difficulties had impacted negatively on the service they provided to students.
Irish, Maths, French, Home Economics, Spanish, and Physics were among the subject areas with most severe recruitment and retention difficulties.
The union has called for an acceleration of the process of pay equalisation for teachers who began employment since 2011.
It said that changing demographics meant secondary schools would require an additional 2,000 teachers over the next six years.
The union said recruitment problems would greatly worsen unless the right actions were taken.