It's expected some classes may have to stay at home.
Schools are reopening across the country today following the Christmas break.
It comes amid concern about the impact high Covid infection rates will have on staffing levels and student attendance.
Teacher unions are warning thousands of staff will be absent today, and it's expected some classes may have to stay at home.
Children with special needs, younger primary pupils and third, fifth and sixth year post-primary classes are being prioritised for in-person teaching.
President of Association of Secondary Teachers' of Ireland, Laois' Eamon Dennehy, says some schools could be forced to close in the coming weeks due to Omicron:
The general secretary of the Teachers' Union of Ireland says he is already hearing reports of staff shortages in schools across the country.
Tullamore's Michael Gillespie added it was too early to predict what may happen: