Update: Dún Laoghaire-Rathdown County Council have said that the orange water at Sandycove Beach today is a naturally occurring micro-alga.
Swimming bans are in place at four beaches in Dublin due to wastewater overflow after recent heavy rains.
Officers took samples of the water at Sandycove this morning and they said that results show it is not raw sewage but is a micro-alga called Noctiluca scintillans.
They said it is "a benign, non-toxic, species" and the ‘blooms’ are usually yellow, orange or orange/red.
They explained that the micro-alga is not directly associated with the wastewater overflow associated with the temporary bathing prohibition, but is a natural summer phenomenon in response to the longer daylight, high nutrients and warm water.
The blooms have been reported along the east coast for the past few weeks and are typical for this time of the year.
The algal bloom exhibits bioluminescence, blue sparkles of light emitted when disturbed by waves on the shore or in the wake of a boat, hence the name Noctiluca, or Night Light.
Swimming is banned at four Dublin beaches for the coming days.
The news comes as a heatwave is forecast for later this week.
Last night Dún Laoghaire-Rathdown County Council said too much wastewater has discharged at several treatment facilities in the area because of heavy rain over the preceding 48-hours.
As a precaution, it's banned swimming at Seapoint, Sandycove and Forty Foot beaches.
This will remain the case until water conditions return to normal and test-results are clear.
It is the second time this month such a ban has been put in place. Eight beaches in Dublin had similar restrictions earlier this month.
Green Party councillor Ossian Smyth says the latest ban is a huge blow to the area.
"We need two things. One is, we need to have Irish Water carry out some remedial before their major project to actually fix-up and replace their wastewater treatment plane. They have to do something to contain it," said Mr Smyth.
It was announced yesterday afternoon that Dollymount was also shut to swimmers after the Ringsend wastewater treatment plant was unable to cope with a low-level rainfall warning.
It was reported yesterday that heatwave could hit the country this week with temperatures rising to the high 20s later in the week.
In Continental Europe, there is the possibility of an 'exceptional heatwave' in the coming days.
Authorities around Paris have issued an orange alert for intense heat, the second-highest level on its scale, with the heat wave expected to last all week with temperatures of up to 40C.
Climatologist Professor John Sweeney said yesterday it is unlikely we will experience such extremes.
"Met Éireann are saying we'll get temperatures up to 27C on Thursday but we won't get, one suspects, the kind of prolonged, very hot temperatures that our near neighbours are getting," said Mr Sweeney.
"Maybe that's no harm because we know that even with temperatures in the high 20s, we notice a mortality peak, even here in Ireland on a smaller scale."