More than 36,000 teachers have yet to be vetted by gardaí, according to figures obtained by RTÉ's Morning Ireland.
Only new teachers and those moving schools are currently required to obtain garda clearance, which is administered by the Teaching Council.
Latest figures show that while almost 90,000 teachers are registered with the council, about 40% have yet to be garda vetted.
New legislation is being drafted that will make it mandatory for people working with children and vulnerable adults to acquire garda clearance, but the Department of Education has confirmed existing teachers will not be immediately subject to the requirements.
In a statement, the department said forthcoming legislation will "focus initially on the vetting requirements in respect of new employees and subsequently on vetting requirements in relation to existing employees".
The National Parents Council has raised concern about the number of teachers who have not been vetted.
CEO Áine Lynch said: "This issue needs to be addressed urgently. Teachers working in the unsupervised positions that they do with children should be garda vetted.
"This group should be prioritised for vetting."
However, she added that there are additional child protection policies in place, which "make sure schools are safe places".
The department is understood to be anxious to avoid overwhelming the vetting system with teacher applications.
A dedicated garda central vetting unit was established in 2002 and strengthened with additional staff in 2006. It processes an average of 6,000 applications every week.
Existing staff at some Education and Training Boards, formerly known as VECs, were required to obtain garda vetting when it was originally introduced.
The Irish National Teachers' Organisation said it would welcome moves by the Government to put additional resources into garda vetting to ensure the process "can take place in the shorter term".
INTO General Secretary Sheila Nunan said: "We don’t want a situation where a school could find itself without a number of its staff being able to report for work.
"Teachers will be vetted over the next number of years and that is something that we fully support."
Ms Nunan said there is already "an enormous awareness among the school community to be 100% vigilant in implementing everything to do with child protection".
Independent TD Denis Naughten, who has campaigned for reform of the vetting system, described the situation as "completely unacceptable".
"In 2006, we were told every teacher would be vetted on a phased basis. We are now eight years down the road and we still have not vetted four out of ten teachers. "
Mr Naughten said many parents were unaware that teachers had not been through the garda clearance process.
The Teaching Council said there are plans to expand vetting arrangements on a phased basis over the coming years.
The average waiting time for garda vetting, according to figures from the first half of the year, is four weeks.
Responding to a parliamentary question earlier this year, Minister for Justice Frances Fitzgerald said an e-vetting system is being developed to streamline the vetting process.
The minister said the move would "greatly contribute to a reduction in processing times for vetting applications".
The Chief Executive of the Children's Rights Alliance has said there is an obligation on the State to ensure that every child is kept safe.
Tanya Ward said the situation needs to be resolved as quickly as possible and extra staff should be provided for a short period of time to avoid a backlog of vetting applications.
Speaking on RTÉ's Morning Ireland, she said: "Vetting is the cornerstone of the child protection system and I think what's really important here is that the Minister for Education, the Minister for Children and Youth Affairs and the Minister for Justice try and work together to resolve this situation.
"You don't want the system flooded. I think what you do need though is an injection of staff for a short period of time to try to resolve it within the coming year."
