Three in every four primary school principals are double-jobbing by teaching and running schools simultaneously, according to the Irish National Teachers' Organisation.
Such overwork is breeding despair and cynicism among principals, who are quitting their careers in greater numbers, according to the INTO general secretary, Mr John Carr.
The union is demanding talks with the Department of Education and Science to ease the workload of principals.
At the INTO principals' conference in Dundalk today, 350 principals will discuss the possibility of several small schools in an area sharing full-time principals and deputy principals.
"Principals teach full time and when that is done they have to turn their hands to running the school," said Mr Carr.
"To have to face this at the end of a day spent teaching full time is unfair and unjust and no longer sustainable. The complex task of running a school, however small, cannot be done on top of a full-time teaching job," he said.
"The net result is that our principals are run ragged and are simply quitting the job in increasing numbers."
Class teaching by principals is suffering from both principals' and pupils' points of view. Principals of small schools in particular feel that the allowance for the role is inadequate for the work and the responsibility, Mr Carr said.
The INTO had secured a small number of days free from teaching for principals, but this was a far from perfect solution.
The bureaucratic demands to fill forms, read reports, devise school plans and policies that were demanded of principal teachers were increasing year on year.
"It is imperative that the time in which to do this work increases at the same rate," he said.
More than half of Ireland's 3,200 primary schools have fewer than 100 pupils. These 1,700 schools serve about one-fifth of the pupil population and mainly in small rural areas.
Many of these smaller schools serve pupils who belong to a religious minority, and in most small schools children are taught in a multi-class situation. Small schools also include new or developing schools, many of which grow as new pupils enrol each year.
The INTO argues that small schools are an essential part of the fabric of rural Ireland. They sustain social well-being, instil fierce loyalty and offer a unique atmosphere in which children can learn.
In the recent past small schools have come under pressure.
The 1995 White Paper, Charting our Education Future, raised questions about the breadth of the curriculum that can be offered in small schools, pointed out limitations in the availability of physical resources and stressed the difficulties in providing serviceslike learning support.
Other studies have challenged the notion that small schools cannot provide as wide and varied curriculum as larger schools.