People have been urged to avoid all non-essential journeys on public transport as schools reopen this week.
The chief executive of the National Transport Authority said Covid-19 restrictions were putting pressure on public transport services and she appealed to the public not to use the service if it is not necessary so frontline workers can get to their jobs.
Anne Graham made the plea as students prepare to go back to the classroom on Monday following the mid-term break.
Public transport capacity has been restricted to 25% of its normal capacity in line with the Government's level five restrictions.
Ms Graham said this had resulted in some services coming under severe pressure with passengers being left behind in some instances.
The problem is most acute on buses early in the morning, particularly in the Dublin region.
"The return of schools on Monday is likely to give rise to additional pressure," said Ms Graham.
"We are anxious to make sure that customers are aware of the situation.
"NTA is asking people to avoid travelling at peak times and only to use public transport for essential purposes.
"Nobody wants to see a situation where essential workers, such as those on the healthcare frontline, cannot get on a bus.
"People who can work from home should work from home."
She encouraged people to walk or cycle where possible.
"Customers who have no alternative to public transport should expect their journey to take longer and plan ahead accordingly," she added.
"Drivers and other public transport staff are working in a very challenging environment, and customers should remain respectful of them at all times."
Ms Graham also reminded transport users to wear a face covering while using rail and bus services.