Authorities have avoided trying to ease traffic chaos for strike-hit Luas passengers out of fear of exacerbating protests at depots around the city, sources have revealed.
When Transdev put on extra buses on St Patrick's Day, the National Bus and Railworkers Union (NBRU) accused it of hiring 'scab labour'.
Sources have said the experience has frightened off those considering extra services.
People Before Profit condemned the move, and sources said members of Eirigi were supporting the drivers.
"There is a fear that things could get ugly (if more buses were put on during strikes)," said a source. "The authorities do not want passengers involved in that situation."
Transport minister Shane Ross and the National Transport Authority (NTA) said they cannot put on extra services for Luas passengers, who have endured 11 strikes and face another nine.
A spokesman for Mr Ross said he was not going to intervene as the dispute was a "matter for the private company" and putting on extra services would be a question for the National Transport Authority.
The NTA said putting on extra services was not feasible and the main constraint was the lack of availability of vehicles, the smaller capacity of buses, and the fact that extra buses would increase traffic congestion.
"When we looked at the practicalities, we couldn't get it to work at all in terms of expectations," said a spokeswoman.
"A Luas carries 250 to 300 people, while a bus carries 80 to 85, and you would need a lot of them. Even if you could magic up the buses, they could end up stuck in traffic."
The Herald has learned that the NTA has powers under the Dublin Transport Authority Act to become the "operator of last resort".
This can be applied if the operator, in this case Transdev, is unable to meet its contractual commitments, withdraws from providing or "fails to provide" services set out in the contract.
Negotiations in the Luas dispute are at an impasse. Drivers want a 23pc pay rise by 2019, but the tram company is offering 13pc.
The drivers' colleagues - ticket inspectors, ticket inspector supervisors, and control room staff - have accepted the 13pc offer.
Meanwhile, the managing director of Luas operator Transdev, Gerry Madden, has said that 13pc pay increases were still on the table for drivers, if they stopped their strike action.
Mr Madden has also promised that a 10pc decrease in wages imposed on drivers for working to rule could be immediately rescinded, if they called off that action.
However, in a carrot and stick gesture, he also warned that the pay penalty could go further.
Yesterday, the drivers staged pickets at Luas depots as the dispute over pay continued.
They have notified Transdev of two further full days of strike this month on May 20 and 27.
Four-hour stoppages are also planned for May 26, June, 2, 3, 7, 8, 9 and 10. Some of the strike days will affect those attending the Bruce Springsteen concert in Croke Park.
Mr Madden said that the offer was still on the table for drivers, if they called off the action, but warned the amount the company could afford to pay was diminishing.
Mr Madden said any deal would have to take consideration of "a further diminishing pot" and more strikes meant "more fees to pay" for the Luas operator.
Luas drivers amended the times of their additional strikes scheduled for June in order to avoid potential disruption to students doing the Junior and Leaving Certificate examinations.
They notified Transdev that the strikes on June 8, 9 and 10 - the first three days of the State examination timetable - will take place from 6pm to 10pm.