Almost four out of five second level schools have had no applications to teaching posts they advertised over the last six months, according to the Teachers' Union of Ireland.
A survey of more than 130 schools carried out by the union found found that virtually all of the schools that responded - 97% - have experienced difficulties recruiting teachers, and almost half complained of difficulties retaining teachers.
77% said they had advertised positions in the previous six months for which no teacher had applied.
More than half of schools that participated said they had unfilled vacancies due to recruitment and retention difficulties.
The findings have been published in advance of a one day strike next Tuesday that TUI members will engage in.
The strike is aimed at highlighting the union's concern at an ongoing disparity in pay for teachers employed after January 2011 and hundreds of post-primary schools are expected to close as a result of the action.
Schools run by Education and Training Boards, where the majority of teachers belong to the TUI, will be forced to close.
But many other schools with teachers belonging to either the TUI or the ASTI are also likely to remain shut for the day. This is because ASTI teachers - who are not participating directly in the strike - will not do work normally done by their TUI colleagues on the day.
The one day strike will also affect adult and further education centres, and Institutes of Technology and Technological University Dublin.
The union said the failure to end pay discrimination is the single greatest cause of the ongoing teacher supply crisis that its survey highlights.
TUI President Seamus Lahart said the findings made clear "the severe damage that the injustice of the two-tier pay system is doing to the education system and the service to students".
The union said it is alarming but predictable that the situation has worsened since the same survey was last carried out in April 2019.
It described as "damning" the fact that almost four out of every five schools advertised positions to which no teacher applied, while over half of schools have unfilled vacancies due to recruitment and retention difficulties.
It said the level of the crisis was unprecedented.
In a statement the union said there was no doubt that teacher recruitment and retention problems were inflicting severe damage, with students often missing out on subject choices or being taught by "out-of-field" teachers.
The survey found that the ongoing teacher shortage is most severe in subjects such as Home Economics, French, and Math.
Other subjects affected include Spanish, Biology, Physics, and Chemistry.
According to the union the largest pay discrimination still occurs in the early years of employment, with new entrants to second level teaching earning 14% less on initial appointment and 10% less in the first ten years than they would have before the imposition of a two-tier pay system.
It says that within the first ten years of their career, those teachers earn over €50,000 less than a teacher appointed pre-2011. Over a 40-year career, they earn around €110,000 less.
The union has called for a commitment to an immediate ending of pay discrimination to be contained in any new programme for government.