One crew member of the missing coastguard helicopter has been found and is in a critical condition.
He is on a lifeboat being brought ashore, however officials say hopes are fading for his three crew members.
The coastguard says debris has been found spread over a wide area off the Co Mayo coast.
They’ve also confirmed no distress call was received from Rescue 116 before they lost contact at around 1am.
It had been helping another helicopter with a medevac operation to airlift a crewman off a fishing vessel, when it disappeared from radar screens.
The Shannon based helicopter, Rescue 115, the Sligo search and rescue helicopter and the Air Corps Casa aircraft, have joined the search operation, several miles out to sea.
Lifeboats and fishing vessels are also involved.
Director of the Coastguard Eugene Clonan says it’s a dark day for the emergency services.