People are being urged to only use public transport for essential journeys as schools return from the midterm break.
There is less space available on many services due to Covid-19 restrictions.
Since level 5 restrictions came into effect capacity on public transport has been operating at 25%.
The National Transport Authority says people should consider alternative ways of travelling rather than public transport.
It comes as the National Bus and Rail Union is a calling for dedicated bus service for healthcare workers.
The NTA is appealing to people to remain respectful to drivers as they are working in challenging conditions.