Education Minister Ruairi Quinn wants explanations from primary schools that are employing retired teachers for long periods.
The retirees are being hired for weeks at a time despite strict regulations, the Irish Independent can reveal.
Now the Department of Education is planning to investigate individual cases to see if schools are flouting the rules.
Principals are under instructions to use retirees only when they cannot find an unemployed graduate to substitute for a teacher on sick or maternity leave.
As a more detailed breakdown on the use of retirees emerged, the Irish National Teachers Organisation (INTO) said it was indefensible given the number of qualified teachers available to work.
Around 1,800 new primary teachers graduate each year – three times the number of jobs that were filled last year.
The Irish Independent sought data under Freedom of Information legislation on the use of retired teachers in the eight-week period up to the mid-term break last October.
Principals argue that the lack of qualified teachers in some places leaves them with no choice but to hire retirees, as it may not be feasible for a graduate to travel a long distance with the expense that involves.
During the eight-week period looked at, there were 237 instances of retired primary teachers employed on a substitute or temporary basis for varying lengths of time in the country's 3,200 national schools.
Fifty-one retirees were employed in the same school for between one and six weeks or more, two covered for six weeks or more, six for four to six weeks, 15 for two to three weeks and 28 for one to two weeks.
The department refused to provide a breakdown of where retired teachers were employed, which would have given further insights into whether a principal could argue that location was a factor.
A spokesperson said that "the cases where retired teachers have been employed for long periods will be examined by the department".
It wants "to determine if there are particular reasons why they have been employed for such long periods and to ensure adherence to the terms of the circular issued by the department in 2011".
The circular also prohibits schools from recruiting anyone in a teaching capacity who is not a qualified teacher.
INTO general secretary Sheila Nunan said there might be "a tiny number" of cases where schools could not get an unemployed teacher and called on a retired teacher at short notice.
However, she said the rehiring of retired teachers on a regular basis or for extended periods was indefensible given the number of qualified teachers without jobs.