Fingal County Council has published a shortlist of sites for a regional sewage plant.
The council says a large plant capable of catering for a population of 700,000 will be needed when the Ringsend plant reaches capacity.
The sites now being considered for the 50-acre facility are Annsbrook and Newtowncorduff near Lusk with an outfall near Loughshinny north of Rush, and Clonshaugh near Dublin Airport with an outfall near Ireland's Eye south of Portmarnock.
Local residents in north Dublin are opposed to the €500m development, claiming that a number of smaller sites would be preferable.
But the council says that one large plant discharging into the Irish Sea is the most sustainable option.