More than 14,000 unqualified individuals were employed as teachers last year.
According to Freedom of Information figures, 8,871 people who were not registered with the Teaching Council worked 136,109 days in a substitute capacity in primary schools.
Meanwhile, 5,145 unregistered individuals worked 91,845 days in a substitute capacity in secondary schools.
These figures exclude schools run by Education and Training Boards.
Schools are entitled to employ unregistered teachers in urgent or unforeseen circumstances, but they are limited to teaching in a particular school for a maximum of five days and are paid an unqualified rate of pay.
Labour Party TD and spokesman on Education, Aodhán Ó Ríordáin, says the situation is unacceptable;
"We can't stand over a situation where our children are being taught by those who are untrained, it undermines the profession, it undermines their education."
A survey carried out by the Teachers Union of Ireland found that 91 per cent of secondary schools reported difficulties in recruiting teachers in the past six months.
The Department of Education says an unregistered person must hold at least a level seven qualification on the National Framework of Qualifications, and any unregistered person who is a teacher must be vetted for child-protection reasons.