Fine Gael has described as "unnacceptable" that extra bus services have not been put in place to allow passengers travel to and from Luas stations.
As the first line of Dublin's new light rail startes its first full week of operation, Senator Brian Hayes said the lack of feeder buses was a "huge oversight on the part of the Minister for Transport."
"We have been told in recent weeks that there is a real possibility that Luas will fail to attract sufficient numbers of passengers, with any shortfall having to be made up by taxpayer's money and given the lack of feeder buses, such predictions are hardly surprising," Mr Hayes said in a statement.
"Minister Brennan must rectify this situation immediately. It is time that he concluded agreements with Dublin Bus or other private bus operators to carry people to and from Luas stations," he added.
The Luas "Green" line began taking passengers last Wednesday. The service has operated free of charge since then, but passengers must begin paying tomorrow morning. The "Red" line from Tallaght to Connolly Station in the city centre is due open at the end of the summer. Luas has cost the Government almost €800 million.