Investigators have confirmed that a crew member of the Rescue 116 helicopter has been located in the aircraft's wreckage but added that they can not confirm his identity this stage.
Senior Gardai and rescue workers added that the two other missing crew members had not yet been located.
The three men- Mark Duffy, Ciaran Smith snd Paul Ormsby- were on board the Irish Coast Guard helicopter when it collided with the Blackrock island on the early hours of March 14.
Speaking at a media briefing at Blacksod lighthouse at 11.30pm tonight, Garda Supt Tony Healy said: "I can confirm that we found the body of a crew member in the cockpit section of the aircraft on the seabed. Operations will continue over night.
"It is not possible to confirm the identity of the crew member at this point," Supt Healy said.
The senior officer also met with relatives of the three missing crew members this evening, and said that they were constantly being updated on any developments.
"It's obviously a very challenging time for the family and they're going through a difficult time and stressful situation waiting for the recovery of their loved ones. They are being fully updated by a garda family liaison officer,"Supt Healy said.
Efforts are currently focused on retrieving the crew member who has been located.
The Granuaile will be stationed near the Blackrock island over night with cover being provided by the LE Samuel Beckett.
An Remotely Operated Vehicle (ROV) will also be on standby overnight.
Earlier, divers recovered the 'black box' recording device of the tragic helicopter.
Diving operations commenced off the Mayo coast in the search for three missing Irish Coast Guard members earlier today.
In a statement on Friday evening, the Air Accident Investigation Unit (AAIU) said the flight recorder will be taken under escort to Baldonnel Aerodrome where it will be onward transported to the Air Accidents Investigation Branch (AAIB) in the UK for download.
Chief inspector with the AAIU, Jurgen Whyte, told Independent.ie earlier that the "most important thing" was to find the 'black box'.
He confirmed shortly after 5pm this evening that the data recorded had in fact been found by divers.
"We're happy to announce the positive recovery of the data recorder at 16:30 today from the seabed just off Blackrock," Mr Whyte said.
"The recorder is now in the possession of one of my investigators, the investigator in charge Paul Farrell, and it's presently located on the Granuaile.
"During the night after some work we'll recover the recorder back to Dublin where it will be flown to the UK Air Accident Investigation branch where it will be further downloaded, hopefully successfully by Tuesday or Wednesday of next week."
Supt Tony Healy said that divers are continuing to operate near the Blackrock island and that further data will need to be analysed to determine if the three missing crew members are with or near the wreckage.
In a statement issued this evening, the AAIU wrote;
"The Air Accident Investigation Unit (AAIU) in consultation with An Garda Síochána, the Naval Service, the Coast Guard, the GSI, the Commissioner for Irish Lights (CIL) and the Marine Institute wish to advise that the flight recorder (black box) of R116 has been successfully recovered from the sea bed on the Eastern side of Black Rock, at a depth of approx. 40m.
"The flight recorder is now in the custody of the AAIU and will be taken under escort to Baldonnel Aerodrome where it will be onward transported to the Air Accidents Investigation Branch (AAIB) in the UK tomorrow, Saturday 25 March for download."
The Naval Service team were deployed shortly after 11.30am this morning from the Irish Light's vessel, the Granuaile.
An underwater robot had been used to carry out an examination of the wreckage of the Sikorsky S-92 helicopter, located in depths of 40m close to the Blackrock island.
Senior investigators stated that they were hopeful of progress being made throughout the day but stressed that information from the divers will need to be assessed before they can establish if the three crew men are with the wreckage.
Speaking at a press briefing this morning, Supt Tony Healy said: "This morning conditions are ideal, diving operations have commenced and we're waiting for reports back from the divers as soon as they come back on the surface."
He added that there was no indication at this stage if the three missing crew members- Mark Duffy, Ciaran Smith and Paul Ormsby- are with the aircraft's wreckage.
Jurgen Whyte of the Air Accident Investigation Unit (AAIU) said that it had not yet been established what caused the Rescue 116 helicopter to collide with the Blackrock lighthouse located 12km off the Mayo coast.
"As we said all along the important thing is that we get access to the cockpit voice recorder and the flight recorder. The priority has always been to recover the victims and if the divers happen to come across the recorder they will take the recorder. Because if we see it, we must take it because we can't leave it behind," he said.
A family liaison officer has been briefing the family members of the missing crew men on any developments in the search operation and investigation.
The Naval Service dive team will operate in pairs and will be able to spend nine minutes at the seabed before returning to the surface.
Air, shore and surface searches are also being carried out, with over 200 personnel from various agencies involved in the large scale operation.