Audits of 13 school building construction sites have uncovered employment irregularities in more than half.
Seven cases have been referred to the Revenue Commissioners and other State agencies for further investigation.
The reviews were carried out on behalf of the Department of Education as a result of complaints received.
The State is in the middle of a massive school building programme, which is worth €2bn.
While the department said the vast majority of construction companies involved are compliant in terms of pay and conditions, it is keen to ensure that all are.
Last year, a private company commissioned by the department completed investigations at 13 sites, which were chosen as a result of complaints made to the department.
Five have been referred to Revenue for further investigation, one to the National Employment Rights Agency and another to the Department of Social Protection.
It is understood this last complaint related to a worker being told he could supplement the wage he was receiving by signing on.
In 2013, a total of 28 complaints were received and related to 22 educational building projects.
Most of these complaints were made through an online complaints system run by the department.
The department said it will investigate allegations of improper practice in the school building programme.