Taoiseach Enda Kenny has extended the "deepest sympathy" of the Government to the family of Captain Dara Fitzpatrick..
Speaking in Washington, Mr Kenny said he knows the area of the Mayo coast where Rescue 116 went down well.
"Obviously the search continues and on behalf of the Government again we pay our respects to the family of Dara Fitzpatrick. The search continues for other crew members.
"Clearly in the analysis and investigation into this we want to uncover what exactly happened in respect of the lack of communications and loss of contact with Rescue Helicopter 116," Mr Kenny said.
He told reporters that he learned about the lack of communications from the rescue team late last night.
The Taoiseach has been briefed on the unfolding situation by Transport Minister Shane Ross and the Chief of Staff for the Defence Forces Mark Mellet.
"The Irish Coast Guard have been exceptionally professional and competent in their work over many years.
"Last year alone over 2,500 incidents were directed by the Irish Coast Guard," he said.
"Obviously the rescue agencies are now coordinating their search in the locality, assisted by the Achill Lifeboat and local fishermen. Obviously Gardaí and the Defence Forces are in the area."
President Michale D Higgins has also paid tribute to the late Coast Guard member.
"Today marks a dark day in the history of the Coast Guard / Garda Cósta na hÉireann, with a member of this important service losing her life while providing assistance to others," he said.
"On behalf of the people of Ireland, may I pay tribute to Captain Dara Fitzpatrick who died today.
"We are all grateful for the courage, resolution and exemplary commitment to the aims of the Coast Guard that Captain Fitzpatrick and her colleagues have consistently displayed.
"My thoughts are with her family at this difficult moment and also with the families of the missing crew.
"It is appropriate that we acknowledge and pay tribute to the efforts of her colleagues at the Coast Guard who have been working at a time, which must be for them, a time of great grief and uncertainty.
"May I also commend the other emergency services, as well as those from local communities in Mayo, who responded so valiantly, and in great solidarity, at this time of tragedy."