The search is continuing for the missing crew
It has emerged the Air Corps was initially asked to provide Top Cover for the mission missing helicopter Rescue 116 was sent on.
Top Cover is when an aircraft provides safety and communication support to another.
The Defence Forces have confirmed to Newstalk Breakfast they could not provide an aircraft outside of normal hours, due to a loss of experienced personnel.
It says: "Top Cover is provided on an 'as available' basis.
"Availability is determined having regard to crew availability, aircraft availability, weather conditions etc."
At 10.06pm on Monday March 13th, the Irish Air Corps received a request from the Coast Guard to provide top cover for a long-range medical evacuation off the west coast.
They replied that it was not possible to provide fixed wing aircraft cover outside of normal hours.
They confirmed to Newstalk: "This is because of a loss of experienced personnel (both Air Crew and Air Traffic Control)".
The Rescue 116 helicopter was subsequently sent to provide Top Cover for the mission.
The Defence Forces say at 1.45am on March 14th, they then received an emergency request to conduct a search for the helicopter - which had gone missing.
They initiated a recall plan and were able to provide a Casa CN235 aircraft to assist.
The Air Corps say plans are in place to deal with the shortages in personnel.
It says there are currently 28 cadets, in three classes, undergoing various stages of Air Corps cadetship to become pilots.
Captain Dara Fitzpatrick died after being recovered from the water on Tuesday, but three crew members - Mark Duffy, Paul Ormsby and Ciaran Smith - are still missing.
It comes as the resourcing and staffing levels of the Air Corps was raised in the Dáil last month.
An Taoiseach Enda Kenny admitted it was leading to the grounding of aircraft.
The Government also admitted that the staffing shortage in the Air Corps has led to reduced availability of aircraft for the transfer of patients to hospitals for transplants.
On February 8th, answering a leaders' question from Gerry Adams, Mr Kenny admitted there was a shortage of pilots in the Air Corps.
Mr Kenny told the Dáil: "The Air Corps has experienced retirements of highly experienced personnel. There is a current shortage of pilots within the Air Corps, which is a central issue, but the Air Corps continues to provide an inter-hospital transfer service, albeit on a reduced basis because of that.
"It means that out of hours on weekdays, between 6.00pm and 8.00am the following day, the Air Corps does not have air crews standing by for fixed-wing aircraft.
"As to what happens in the meantime, obviously every effort is being made to address this", he said.
Mr Kenny also said that the shortage was partially due to pilots who have retired, adding: "We do not have control over that".
The other main factor, Mr Kenny said, was air traffic controllers.
"Given the technology, it is possible to carry out air traffic control from different locations.
"There are 28 cadets currently in training who will become pilots who are able to fly both fixed-wing aircraft and helicopters.
"In the meantime, outside the hours I have mentioned, we must use either the Coast Guard, the Custume Barracks emergency aeromedical support or private enterprise", Mr Kenny added.
While on February 15th, in written replies, Deputy Thomas Broughan asked about the number of vacancies in the Air Corps.
He was told by Minister of State at the Department of Defence, Deputy Paul Kehoe, that there are 26 vacancies for pilots - with 107 currently serving.