There is free travel for all under-fives on public transport from today.
It is part of the fare changes announced earlier this year by the National Transport Authority.
NTA CEO Anne Graham says it is a step towards making transport a more affordable option for more families.
"There is free travel for all under-fives on subsidised public transport services, starting today," said Ms Graham.
"As part of our determination earlier this year, we announced that all under fives get to travel free on Bus Éireann, Iarnród Éireann, Luas, Dart and Dublin Bus services."