The discrimination within the two-tier pay system in schools is having a deeply unsettling effect on the teaching profession, the Teachers' Union of Ireland (TUI) has said.
TUI general secretary John MacGabhann said the lower pay is driving a significant number of people away from the teaching profession.
More than 400 second level schools closed on Tuesday as part of the union's strike action against unequal pay.
Mr MacGabhann said that Tuesday's action is one day of their campaign and pledged to continue until the two-tier pay system, in place since 2011, is removed.
The union said that teachers and lecturers who entered the system since 2011 are paid at a lower rate than their colleagues for carrying out the same work.
"People are fed up and out of patience and want more than what they are getting," Mr MacGabhann said.
"We were told by the Minister (Joe McHugh) last April that he would move to resolve the issue and that he would do so quickly.
"His definition of 'quickly' differs significantly from ours.
"The discrimination is having a deeply unsettling effect on the teaching profession and is driving a significant number of people away from the profession and driving people who have been in the profession out of it.
"Parents are realising the consequence of that for their children, for their grandchildren, is that subjects are lost, services are fractured, and you have a succession of teachers for your children rather than the stability of one teacher.
"If you require teaching by specialists, be it chemistry or maths or technology, you are getting instead people who are doing it just to keep classes going.
"What you have is a deterioration in the quality of services provided to students and this is a direct consequence of the teacher supply crisis and that is a direct consequence of the inequality at play."
Mr MacGabhann did not rule out further strike action, stating that today is one day in their campaign of action.
"This is not the end of the campaign, this is a stepping stone," he added.
In a statement, the Department of Education said: "On 24 September 2018, an agreement was reached between the Government and the public services committee of ICTU in respect of new entrant pay.
"This agreement is benefiting 18,000 teachers and nearly 5,000 SNAs (special needs assistants) within the education sector with a series of incremental jumps for new entrants.
"The teacher unions have indicated that they have outstanding issues of concern.
"The Government gave a commitment that these matters will be given full consideration and that commitment remains.
"The current series of restorative measures for new entrants have been achieved through continued engagement and collective bargaining between the Government and the public service unions and shows the benefits that such engagement can bring.
"The matter of new entrant pay is a cross-sectoral issue, not just an issue for the education sector.
"The Government supports the gradual, negotiated repeal of the FEMPI legislation, having due regard to the priority to improve public services and in recognition of the essential role played by public servants.
"The starting salary for a new entrant teacher in 2012 was 30,702 euro. The starting salary of a teacher is now 36,953 euro.
"From 1 October 2020 onwards, the starting salary of a teacher will be 37,692 euro."
Fianna Fail's spokesman on Education, Thomas Byrne, said: "Today's strike is a major inconvenience for thousands throughout the country and indeed a significant step by teachers.
"It illustrates one of the issues that comes up on the doorsteps - the crisis between the Government and the public sector.
"In area after area, we are seeing conflict with the people we rely on to deliver public services."
He accused the Government of allowing the conflict to fester until it became an open dispute.
