A legal challenge has begun against the extension of the Ringsend sewage treatment plant.
People living in the area want the planned €270M project to be declared "void".
Under the extension plans, a 9 kilometre pipe will carry treated human waste into Dublin bay.
Dublin City Council was given permission to start works last year - just weeks before an area of the coast from Howth to Dalkey Island was designated as a special area of conversation.
Local residents claim the project is now void, because the correct environmental assessments weren't carried out.
The case, being heard at the Commercial Court, is due to last less than a week.