An Bord Pleanala is being asked to prioritise a decision on a planned expansion of Merlin Pumping Station.
The project would boost its wastewater capacity, which would enable the development of thousands of new homes.
City planners approved the plans last September, but An Taisce took a case to An Bord Pleanala.
A decision is now overdue - and Councillor Alan Cheevers says the city cannot afford to wait.