Iarnród Éireann and Bus Éireann are operating full services today, although they have advised customers to travel only "if really necessary".
Dublin Bus is also operating a full service, and says people who normally use it should continue to do so.
Today's "No Fares Day", which has its precedent in similar protests in Europe, is aimed not just at highlighting trade union grievances over the Government's privatisation plans but at showing how a fully integrated public transport system would work in Ireland.
CIÉ companies fear, however, that commuters may take advantage of the action, with Irish Rail seen as particularly vulnerable in this regard.
Both it and Bus Éireann noted "any increase in the number of people travelling, over and above normal levels of demand, will result in capacity issues and difficulty in continuing to provide a service". They added it might be necessary to implement "loading control procedures" at busy stations in order to accommodate people with pre-paid tickets.
Dublin Bus said inspectors would be on duty to monitor passenger numbers and help to control them if needs be. "We are not dissuading people from travelling, just saying that whatever people normally use to keep using it," said a company spokeswoman.
Iarnród Éireann's information line, at 1850 366222, is open from 6 a.m. to 6 p.m. today.