Shortly after 6pm, the Air Corps PC-9 two-seater training aircraft was reported missing near Crumlin, north of Lough Corrib, on the Galway-Mayo border.
Late last night, Defence Forces and emergency personnel located the wreckage and confirmed the deaths of the crewmen.
Their remains were at the scene of the crash, which has been sealed off in a 2km cordon by gardaí.
The crash site will be examined at first light today by air accident investigators from the Air Corps and Department of Transport. The remains of the two crew can only be removed after the scene has been cleared of the explosives from the ejector seats, which had not been activated.
The names of the dead crew members were not released.
Last night, the Chief of Staff, Lieutenant General Dermot Earley, and the General Officer Commanding the Air Corps, Brigadier General Ralph James, expressed their deepest sympathies to the families of the airmen.
The aircraft was carrying a flight instructor and cadet involved in a training exercise, as were two other planes from Casement Aerodrome, Baldonnel, at the time. The planes took off at 5.45pm, but a short time later one of them got into difficulty. All three planes were on separate exercises and did not have visual contact with each other.
Local people reported seeing the aircraft in difficulty and attempting to make an emergency landing in mountainous terrain.
At around 6pm, a woman was reported to have telephoned the air emergency centre in Shannon to report that a plane had gone low overhead and that she heard a loud bang. The two other aircraft returned safely to Galway airport, Defence Forces spokesman Comdt Gavin Young said.
Defence Minister Willie O’Dea said: “I am deeply saddened and shocked to learn of the tragic deaths of the two Air Corps personnel. I wish to offer my sincere condolences, and that of the Government, to the families of the two deceased men at this very sad time. They are in our thoughts and prayers.”
The crash site was about five miles west of Cong, Co Mayo, Comdt Young said.
The Irish Aviation Authority (IAA) was informed of the accident involving the Air Corps Pilatus PC-9.
Emergency services, including a Coast Guard helicopter, ambulances and fire service personnel were at the scene.
Local people helped the emergency services locate the crash site and the area was sealed off.
The aircraft was seen disappearing from sight into a rugged mountainous valley area about three miles from the village of Cornamona.
Locals set out on foot, fearing that it had crashed and some time later located the wreckage of the aircraft at Crimlin East.
People were able to direct the Sligo-based search and rescue helicopter to the scene, while Coast Guard units from Cleggan and Costello Bay also arrived to lend assistance.
Gardaí and fire brigade personnel had to make their way across rugged terrain to reach the crash site.
Local man Noel Mannion said that the plane came down in an isolated area.
“Everybody here just went straight into rescue mode and people first of all identified the exact location for the emergency services and lent whatever help they could.
“Emergency arc lighting was erected as it was very dark. The exact spot where the plane has come down is a glacier valley. It is uninhabited and only used by sheep for grazing.
“People were speaking of the aircraft flying low earlier in the afternoon.
“It is a terrible tragedy for those involved. Our thoughts are with their families,” said Mr Mannion, who is from Cornamona.