Three members of the missing crew from Rescue 116 have been named locally as Mark Duffy, Ciarán Smith and Paul Ormsby .
The Coast Guard members are all experienced crew members.
Mark Duffy (51) is from Blackrock, Co Louth and Mr Smith is originally from north county Dublin.
Father of two Mark Duffy joined the coastguards in 2002 while living in Blackrock, Louth with his wife Hermione.
A much-respected man in his community, Mark once surprised son's school in 2015 by arriving in the coast guard's helicopter.
St Francis National school expressed their gratitude to him on their website.
"There was great excitement in school today when the Irish Coastguard came for a return visit, following their water safety talk in school two weeks ago.
"Our friends at Blackrock Park kindly gave permission for it to land there, and the whole school went to the park to watch it land.
"Each class had the opportunity to go up to the helicopter and look inside. Many thanks to parent, Mark Duffy, a helicopter pilot with the Irish Coastguard, for organising such a fantastic surprise for the pupils of St Francis."
Mark was also involved in the helicopter rescue over the weekend, helping to save the lives of a father and son.
Mark Dearey, the chairperson of Dundalk Municipal District, knows Mark Duffy and his wider family.
Cllr Dearey said “Whatever the outcome is we are proud of him and people think of Mark and his colleagues are heroic. We are so aware that he put himself in danger to save others,” Cllr Dearey added.
He said, “despite ourselves we continue to hope against hope that there might be a happy outcome.”
Mary Moran, a teacher and former Senator said this evening that the pilot as "an absolute gentleman".
"We are absolutely devastated for his wife, his children, his mother and brothers and the whole family.”
“Everybody is in a state of shock," she added.
A source said a St Patrick's Day parade due to take place in his hometown has been cancelled.
In 2014, the Irish Independent featured the lives and seaside home of Mark and his wife.
"In 2002, Mark got his dream job as a coastguard pilot back in Dublin, and they opted to base themselves again in Dundalk, near Hermione's dad, who lives in Omeath, and her mum in Gormanston," the story read.
"Mark's work as a helicopter pilot led to a job for him in the west, and Hermione became pregnant with her daughter – so she closed the business.
"Over time, Hermione started doing up houses, combining rearing the children – she also has Fionn, nine – with interior design."
Dundalk Cllr Emma Coffey expressed her shock at news of the accident.
"It was a terrible tragedy. I can't even comprehend what the poor families are going through at the moment, along with the Coast Guard. We take their work so much for granted - they're constantly putting their lives on the line. I live in Dundalk but work in Drogheda and everyone is talking about it."
Winchman Ciarán Smith (38), from Oldtown in Dublin, is a father-of-three. He is involved in his local GAA club.
Mr Smith lives in Oldtown, in north county Dublin, along with his wife Martina and their three young daughters.
Most of the locals in the small village know Mr Smith well and revealed he is a dedicated family man.
He is a sports fanatic and is heavily involved in his local Wild Geese GAA club in both football and hurling.
A keen cyclist, he is also known in his community for raising funds for charities close to his heart through participation in charity rides.
"It is just absolutely shocking. I know Ciaran's family is still holding out for the small hope he is going to be found alive.
"Ciaran is a great man, an absolute gent. Now I just feel numb," one close friend said.
"I would be very close with Ciaran. They are still searching for him and he's still missing, so out of respect for his family I don't want to say to much at this time."
Mr Smith is a regular at Kettles Country House Hotel, where he often eats lunch with his family.
Locals there were in disbelief about the events that have have unfolded off the west coast.
"He would often he in here with his wife and the three little girls. He is very popular," another local said.
The brave rescue worker and keen cyclist completed the 2,000km Race Around Ireland event in 2015, coming in sixth place.
He undertook the grueling seven-day challenge for charity, raising €10,000.
"He has always been a very selfless man and he has always loved a challenge as well.
Paul Ormsby (53) is the third crew member who has been named as missing this evening.
Paul joined the CHC in 1998 and has nearly two decades of experience as a winch operator.
Paul received commendations for a rescue in 2000 of 17 Spanish and Portuguese sailors who got into difficulty off the coast of Clare in 2000. His rescue team winched 13 of the sailors from their stricken vessel before depositing them safely in the nearby Shannon base.
The search for the missing crew is continuing in Co Mayo.
Resources from the navy, gardaí and local coast guard units were deployed to the scene today as part of the large scale search.
Captain Dara Fitzpatrick, from Kilternan in Co Dublin, was rescued earlier today in critical condition but later died in hospital.
Captn Fitzpatrick was an extremely experienced pilot who had almost 20 years service with the Coast Guard.
She was a high profile water safety campaigner and often spoke publicly about her love for her challenging job.
She is survived by her two year old son.
Her family have led tributes to her today.
"We are so proud of Dara's work and all she has done to save the lives of others over the years," they said in a statement to Today FM.
"We are completely heartbroken and we pray for the recovery of the other three crew.
"She is survived by her three year old son, her three sisters, brother and her parents."
Her sister, leading psychologist Niamh Fitzpatrick, also paid tribute to her on Twitter this afternoon.
"My brave sister Capt Dara Fitzpatrick lost her life. We are devastated. Please pray for recovery of 3 remaining crew".
Tributes have poured in from across the country for Captn with President Michael D Higgins describing the tragedy as a "dark day" for the Coast Guard.
"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," he added.