The pop star has described 31-year-old Niall Doherty as "a lovely young man who I have nothing but fond memories of".
Mr Doherty, from Roscrea, Co Tipperary, died on Sunday along with trainee pilot Damien Deegan (31) when their light aircraft crashed near Birr, Co Offaly.
He was also a childhood friend of a Roscrea doctor who was killed in the Air France tragedy in 2009. Today chart-topper Una said she was "shocked and so saddened to hear the tragic news about Niall Doherty".
"He was a lovely young man who I have nothing but fond memories of. My thoughts and best wishes are with his family and friends at this very difficult time," she added.
Niall's family run a popular hardware store in Roscrea town and his parents Michael and Eileen described their son as a hard worker who looked after all the forward planning of the business. "He was loving, caring and helpful all the time. He was the best son anybody could have wished for," said Eileen.
It also emerged that Niall was a close childhood pal of Dr Aisling Butler (26), who was killed when Air France flight 447 crashed in 2009 en route from Rio de Janeiro to Paris in stormy weather.
Dr Butler's mother Evelyn said it was "surreal" that Niall had died in such similar circumstances to her daughter three years ago. "There wasn't a bad bone in Niall's body," she said.
Mechanical failure is being investigated as a possible cause of the crash.
The funeral Mass of Damien Deegan will take place on Thursday morning in St Brendan's Church in Birr at noon.
His remains will be buried afterwards in Clonoghill cemetery.