They were returning after a night socialising with friends and were crossing the stretch of water that had become swollen in bad weather. There were gale force winds and water levels were high.
The dead pair had been friends for years and worked for a period in New Zealand. They were both single but had large family circles who are well-known in the area, a relative said.
Brook Nettleton, director of Bluegass Contracting in the Te Poi/Matamata area told the Herald that Davitt worked at the company for four years.
"He was an amazing operator and excellent employee," she told the Herald.
"Declan always had a smile on his face and was a friend to everyone. We are in deep shock and so saddened by this news.
"An example of how thoughtful this young man was was when our son was born he organised his mother in Ireland to knit our baby a jersey, hat and booties. He was extremely thoughtful and caring."
Initial reports suggested the vehicle became submerged as it crossed the river and was then carried downstream.
Their bodies were found on Monday afternoon after a major search operation involving Garda divers and the Coastguard.
The vehicle entered the water near Louisburgh at about 2.30am on Monday (UK time).
The vehicle was fully submerged as it was swept away. The man who managed to escape raised the alarm at about 3am.
The two men's funerals will be held later this week.
Public notices said they would be deeply mourned by their "heartbroken" families.
Gardai were continuing to investigate the cause of the incident and post-mortems were due to be carried out.
The Louisburgh Macra Na Feirme, a rural youth organisation, said it was an: "Unbelievably numbing day today as we learned that these two fine men tragically were lost in an accident last night.
"Two valued members since the beginning since re-establishing."
Mayo county councillor Christy Hyland said the victims were from well-known and respected families in the area.
Irish police divers pulled the two bodies from the river at about 4am on Christmas morning.
The vehicle became totally submerged at about 2.30am four miles from Louisburgh, a small town on the southwest corner of Clew Bay.
The teenager raised the alarm at about 3am.
Irish police, coast guard and fire services personnel coordinated a rescue effort.
A spokesperson for the Irish police said yesterday: "Both bodies were recovered and have now been transported to the University Hospital Castle Bar where a post mortem is scheduled to take place tomorrow."
The Coastguard Rescue helicopter R118 and a team from nearby Westport helped in the operation.
