Transdev has insisted the transfer was not in response to the issues dogging the Green line
The Transport Minister Shane Ross has confirmed that seven Luas trams have been transferred from the Red Line to the Green Line to provide additional capacity.
The Green Party has claimed the National Transport Authority (NTA) is "robbing Peter to pay Paul".
Its representative in Dublin South Central, Ed Davitt, said that it was a shocking indictment of poor planning of the Cross-City rollout.
"It is completely unacceptable that Red Line commuters should suffer lower capacity, and a less reliable service due to the poor planning for the rollout of the Cross-City service.
"The solution to alleviating problems on the Green Line, is to create problems on the Red Line?
"That’s not a solution - that's robbing Peter to pay Paul.
"Commuters, on the Red Line and on the Green Line, deserve better than this. The Government and the NTA must get on top of this issue, and quickly."
In a statement, the NTA said the process of moving the trams began last September as part of advance planning for the roll-out of the new cross city service.
The extended line has been dogged with problems since its official unveiling in December - with commuters complaining of longer wait times and slower services.
Passenger numbers on the line have increased by over 28% since the unveiling.
An NTA spokesperson said the decision to move the trams was taken before the new line opened and was "certainly not a response to issues that have arisen in recent weeks on the Green Line."
He said the impact of the transfer on the Red Line service has been "minimal" with enough vehicles still in service to cover peak requirement.
"Transferring trams from one line to the other as needs arise makes sense if there is a tram shortage on one line, and some spare tram capacity on the other," he said.
The Luas Cross-City also included the purchase of seven new, longer trams.
Two of these 55m trams entered service in February - but have been withdrawn due to an "unforeseen technical fault".
A specialist team from France have been flown in to resolve the electrical problem.
Minister Ross says all of the longer trams should be put into service progressively between now and end of May.