Residents in Dublin's Ringsend and Sandymount are apparently being "driven demented" by a fly infestation.
Dublin City Council is trying to find out what is causing the swarm.
It has forced homes and businesses to keep their windows shut for a number of days.
Labour Senator Kevin Humphreys says there could be a number of reasons for the extreme influx.
He said: "The city council has been trying to ascertain where the problem originated from, because within the area we have one of the largest waste water treatment plants.
"It's happened before where algae on the beach has caused problems like this before, but at the moment we're not quite sure."
In a statement, the operators of the new Poolbeg incinerator said the facility was not the source of the problem as "there are no waste deliveries to the plant at the present time".
Senator Humphreys said it was never like this and described what one local resident had to do to avoid the flies.
He said: "One lady has said to ne that she had to lie down and put a book over her face, because there was literally so many flies in her home that had come in and she was extremely distressed by it."