The Coast Guard director has said the hopes of finding the remaining crew who went missing off the west coast of the country "are now fading".
Speaking at a press conference this morning, Eugene Clonan described it as a "dark day for emergency services and the Coast Guard".
One casualty was recovered this morning and is in a critical condition.
"We don't hold a lot of hope for that person," Mr Clonan said.
Three of the four crew who were on board the Coast Guard helicopter are still missing.
Mr Clonan said that the first search and rescue team discovered debris on the surface off the coast of Mayo.
He said they lost contact at about 12.45am as the helicopter was approaching Blacksod to refuel.
"She said she was due to land shortly but she didn't arrive, we carried out a communications search and declared a mayday."
He explained: "The first search and rescue was Rescue 118 from Sligo and she discovered wreckage on the surface about one and a half miles south of Blackrock lighthouse, which is about six miles off Blacksod."
He added that winds at the moment are force six with good visibility.
"At this particular point in time, with the amount the vessels we have out there, our hopes are fading of finding the remainder of the crew.
"Weather conditions yesterday, the visibility wasn’t too good but it was okay to carry out that particular operation."
Mr Clonan said they have a "wide field of debris" to search through.
"We are concentrating on the search at the moment...we still have no idea what happened."
He confirmed that the crew are all from the Dublin-based rescue team but he did not want to disclose their identities as family members are still being contacted.
It is still unclear what happened to the helicopter. The search operation is being coordinated by Malin Head control centre.
The crew were sent out to help a UK registered fishing vessel after reports came in that a crew member was injured.
"We routinely send the Dublin based helicopter to chase incidents on the west coast.
"The search teams have been working tirelessly to coordinate the response to this incident".