The company that runs the Luas, Transdev, has said it will now continue its legal options after Luas drivers escalated their strike action.
Eight more work stoppages are planned over the course of this month and next.
The strike over pay has seen thousands of commuters disrupted in recent weeks, and Transdev says it loses €100,000 for each day of action.
Drivers turned down a reported offer of an 18.7% pay rise last month.
CEO of Dublin Town Richard Guiney said the impact of the dispute is also being felt by business people in Dublin.
"16% of people who come in in shop in Dublin city centre will use the Luas, so obviously there's a possibility that we may lose some of that trade," he said.
"We estimate the cost roughly between €1m to €1.5m per day of a Luas strike, so obviously the longer this goes on, the greater concern it is for the business community."
Five 24-hour Luas strikes are planned for:
- Thursday April 28;
- Wednesday May 4;
- Friday May 13;
- Friday, May 20;
- Thursday May 26 (four-hour stoppage);
- Friday, May 27.
Owen Reidy, Siptu negotiator, said: "The reason why we did this today and gave such lengthy notice is to create the space and opportunity for talks in the hope that the strikes won't have to happen."
But the union representative said there had been no contact from industrial relations mediators at the Workplace Relations Commission since talks broke down before Easter.
Mr Reidy described relations between workers and management as fraught, but said the union hoped to meet management next week in a bid to avert further strikes.